The 21-year-old midfielder, who has previously enjoyed spells at Notts County, Yeovil and Portsmouth, will go straight into Barker's squad for the match at Crewe on Saturday.
"I'm looking forward to getting started," he told the club's official website. "I couldn't wait to get out on loan. I was sort of stuck at Chelsea for a while. I just needed to get out and play some games. It's a great club so I'm relishing the chance to get some time on the pitch.
"I want to get some experience and help Crawley up the table, but I know I will have to bide some time because there's some good players here. When I do get a chance hopefully I will take it and score a few goals because I see myself as a box-to-box midfielder."
Clifford links up with fellow Irishmen Mark Connolly and Billy Clarke at the club, and he says he spoke to Connolly ahead of the move.
"I know Mark as I've played with him as part of the Republic of Ireland set-up in the under-21s," he added. "I spoke to Mark on Wednesday when I heard about Crawley's interest and he couldn't speak more highly of the club and the gaffer."