Hammer possibly left out as well (3)
I believe the answer is:
too
'as well' is the definition.
('too' can be a synonym of 'as well')
'hammer possibly left out' is the wordplay.
'hammer possibly' becomes 'tool' (hammer is a kind of tool).
'left' becomes 'l' (common abbreviation).
'out' is a deletion indicator.
'tool' with 'l' taken away is 'TOO'.
(Other definitions for too that I've seen before include "Unduly" , "Also; overly" , "On top - also" , "Furthermore" , "Likewise" .)