Monkey's endless enthusiasm (5)
I believe the answer is:
verve
'enthusiasm' is the definition.
(I've seen this in another clue)
'monkey's endless' is the wordplay.
'monkey' becomes 'vervet' (vervet is a kind of monkey).
'endless' means to remove the last letter.
'vervet' with its final letter taken off is 'VERVE'.
(Other definitions for verve that I've seen before include "Great liveliness or enthusiasm" , "Dashing enthusiasm" , "Life" , "Root tasting of oysters" , "Sparkle" .)