Hammer slowly or fast (7)
I believe the answer is:
rapidly
'fast' is the definition.
(I know that fast can be written as rapidly)
'hammer slowly' is the wordplay.
'hammer' becomes 'rap' (both can mean to hit).
'slowly' becomes 'idly' (idly can mean slowly or without urgency).
'rap'+'idly'='RAPIDLY'
'or' acts as a link.
(Other definitions for rapidly that I've seen before include "right off the bat" , "Fast" , "With great speed" , "Speedily" , "At a great rate" .)