Posh failed to make the most of several first-half chances and, after Craig Mackail-Smith rattled a post in the third minute, George Boyd was denied by Leon Johnson whose full-length dive and what looked suspiciously like handball went unpunished.
Wycombe had their moments at the other end with John Sutton twice squandering good openings.
The one-time Spurs youngster failed to hit the target after catching Shwan Jalal yards off his line and the Baghdad-born keeper also pounced on a Sutton shot at the near post.
The biggest cheer of the afternoon came when popular defender Guy Branston, recalled from a loan spell at Rochdale, replaced the injured Claude Gnapka.
The loss of the adventurous Frenchman did not curb home aggression and Posh grabbed the lead three minutes into the second half with Boyd firing low into the far corner of the net after Wycombe had been stretched by a surging run from Adam Newton.
Wycombe were level inside six minutes with substitute Matt Bloomfield netting after being sent on by manager Paul Lambert in a bid to give his side more width.
It worked like a charm and, following sustained pressure in which Russell Martin fired a free-kick against the bar and Jalal twice saved at point-blank range from Sutton, Bloomfield - who replaced one-time Posh man Scott McGleish - beat Jalal with a controlled left-footed effort.
Boyd hit the foot of a post when it looked easier to score and home substitutes Chris Whelpdale and Rene Howe both fired high over the bar.
Just when it seemed that the points would be shared, McLean was first to react to an 87th-minute long throw by Whelpdale and he gave goalkeeper Frank Fielding no chance with a fine header.