Saw, perhaps, not entirely as well (3)
I believe the answer is:
too
'as well' is the definition.
(both can mean 'also')
'saw perhaps not entirely' is the wordplay.
'saw perhaps' becomes 'tool' (saw is a kind of tool).
'not entirely' means to remove the last letter (not all of the word).
'tool' with its final letter taken off is 'TOO'.
(Other definitions for too that I've seen before include "As well, also" , "Excessively" , "Additionally" , "Moreover, in addition" , "As well; in addition" .)