Popular girl punches muscular fool (6,5)
I believe the answer is:
string along
'fool' is the definition.
(I've seen this before)
'popular girl punches muscular' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'girl' becomes 'gal'.
'punches' indicates putting letters inside (punch can mean to create a hole in).
'muscular' becomes 'strong' (similar in meaning).
'in'+'gal'='ingal'
'ingal' inserted into 'strong' is 'STRING ALONG'.
(Other definitions for string along that I've seen before include "Stay with" , "Appear to be in agreement with" , "Mislead (someone) over a period of time" , "fool" , "Over time, deceive" .)