Monkey endlessly makes a racket (4)
I believe the answer is:
scam
'a racket' is the definition.
(thesaurus)
'monkey endlessly' is the wordplay.
'monkey' becomes 'scamp' ('scamp' can be a synonym of 'monkey').
'endlessly' means to remove the last letter.
'scamp' with its last letter taken away is 'SCAM'.
'makes' acts as a link.
(Other definitions for scam that I've seen before include "Con trick" , "Swindle (sl.)" , "Swindle (Slang)" , "A modern kind of swindle" , "ploy" .)