Part of the wall fell (3)
I believe the answer is:
hew
'fell' is the definition.
(both can mean to chop)
'part of the wall' is the wordplay.
'part of' indicates a hidden word.
'HEW' can be found hidden inside 'the wall'.
(Other definitions for hew that I've seen before include "Chop or hack at" , "Chop (wood)" , "Shape wood, as with an axe" , "'Cut, fell or shape (3)'" , "Chop or cut with axe" .)