Mclean took his season's tally to 29 with the first two goals, with Craig Mackail-Smith and substitute Scott Rendell completing the scoring after Martin Butler had pulled one back.
The opener was made by Tom Williams who picked out Mclean with a precision pass and the striker finished clinically, side-footing home with his right foot.
Dean Keates saw a left-foot shot pushed over the bar by Phil Barnes and when Craig Morgan beat the keeper from a corner, Nathan Jarman was on hand to clear off the line.
Posh went two up with a well crafted goal as Mackail-Smith delivered a ball to Mclean who hit a right-foot volley past Barnes.
However, Butler pulled a goal back when he connected from close range after Jarman had headed on a Ryan Bennett free-kick.
Peterborough made the game safe when Mclean released Mackail-Smith on the left and he squeezed the ball past Barnes from a tight angle before Rendell made it four with a superb right-foot volley from a Williams cross.