Left primate with small mistake (5)
I believe the answer is:
lapse
'mistake' is the definition.
(lapse is a kind of mistake)
'left primate with small' is the wordplay.
'left' becomes 'l' (common abbreviation).
'primate' becomes 'ape' (I've seen this before).
'with' means one lot of letters goes inside another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'ape' enclosing 's' is 'apse'.
'l'+'apse'='LAPSE'
(Other definitions for lapse that I've seen before include "Slight error as caused by inattention" , "Backslide" , "Become void, expire" , "falling away" , "Error; expire" .)