Small primate (6)
I believe the answer is:
'small primate' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for monkey that I've seen before include "Naughty child; primate" , "Small primate" , "Primate; five hundred (slang)" , "Mischievous type" , "Long-tailed primate" .)