Understanding endless complaint (4)
I believe the answer is:
grip
'understanding' is the definition.
(thesaurus)
'endless complaint' is the wordplay.
'endless' means to remove the last letter.
'complaint' becomes 'gripe' (I've seen this before).
'gripe' with its final letter taken away is 'GRIP'.
(Other definitions for grip that I've seen before include "Grab" , "Hold tight" , "Grasp tightly" , "Clench" , "Catch" .)