A 1-0 victory saw the Rams close the gap on QPR and second-placed Burnley to a single point, with Harry Redknapp's side missing the opportunity to move into the top two by going down to John Eustace's first-half header.
Derby have surged up the table since McClaren took over in September and now sit in fourth place but the manager insists the play-offs remain the target, despite a "very satisfying" victory at the iPro Stadium.
"I said before the game it would be a test of how far we've come. We were ready for a big win and this was a big win. We hadn't beaten any of the top six and this was a challenge.
"We could have killed it a lot earlier. We had chances on the break and we didn't take enough care on the ball. But the clean sheet got us the win.
"That is the most satisfying aspect because in the last two games we've conceded five goals and that's not good enough. The key is to get the balance right. Clean sheets at this stage of the season will get you wins.
"I'm delighted for John Eustace. We needed the first goal. That gave us something to hang on to, especially in the last 15 minutes. It's very satisfying."
McClaren still played down Derby's promotion hopes when he was asked whether they were contenders to finish in the top two.
He said: "I still say no because we haven't got the experience. We've got to show more consistency, especially against the big teams.
"We're still looking at seventh position. That is our target at the present moment. The likes of Leicester, QPR and Burnley have got all the experience."
The game's decisive moment came after 20 minutes.
Rob Green came to punch away Will Hughes' inswinging corner but the ball merely brushed his fist and was diverted into the vicinity of Eustace, who looped his header back into the far top corner of the net.
"Gary O'Neil said he was fouled," said QPR boss Redknapp.
"I haven't seen it again yet though, so I could be talking rubbish, I don't know. But the ref has given (the goal) now, hasn't he?
"We didn't create nearly enough chances, that was the problem.
"We didn't press them in the first half and we seemed more worried about them.
"We were better in the second half but we didn't created enough."
QPR lack firepower with leading scorer Charlie Austin and Matty Phillips both out injured for the remainder of the season.
Redknapp may pin his hopes on Manchester United youngster Will Keane to fire his side back to the Premier League.
The 21-year-old forward has yet to feature since arriving on loan at the end of January but Redknapp thinks he is an exciting prospect.
"Charlie was our goalscorer wasn't he, really?" he said.
"He's a big loss, and Matty Phillips is out for the rest of the season too.
"I may look at it (the loan market) but we've got Kevin Doyle, although he's been out of the goals at Wolves.
"But I'm looking forward to seeing young Will now, he's a good player and he will get a chance.
"It's an area we might be a bit short in but I still feel we have a big chance (of promotion)."