Smart case for a tool (7)
I believe the answer is:
cleaver
'tool' is the definition.
(cleaver is a kind of tool)
'smart case for a' is the wordplay.
'smart' becomes 'clever' ('clever' can be similar in meaning to 'smart').
'case for' means one lot of letters goes inside another.
'clever' enclosing 'a' is 'CLEAVER'.
(Other definitions for cleaver that I've seen before include "Butcher's chopper" , "Butcher's knife" , "Hatchet for chopping meat" , "A clever sort of axe" , "dangerous object?" .)