Understanding endless complaint (4)
I believe the answer is:
Here is my best explanation:
'understanding' is the definition.
'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 "Hold tightly in hand", "Firm hold or travelling bag", "Clasp", "one on film set", "Catch", "Hold tight", "Clench", "Fascinate", "Member of film crew" and "Travelling bag - take firm hold".)