Left primate with second mistake (5)
I believe the answer is:
lapse
'mistake' is the definition.
(lapse is a kind of mistake)
'left primate with second' 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.
'second' becomes 's' (common abbreviation).
'ape' enclosing 's' is 'apse'.
'l'+'apse'='LAPSE'
(Other definitions for lapse that I've seen before include "Revert (to bad habits, etc)" , "Brief failure (of concentration)" , "Sepal (anag)" , "Brief (memory) failure" , "Passing - mistake" .)