Fast lady - rip-off (7)
I believe the answer is:
rapidly
'fast' is the definition.
(I know that fast can be written as rapidly)
'lady rip-off' is the wordplay.
'off' indicates anagramming the letters.
'lady'+'rip'='ladyrip'
'ladyrip' anagrammed gives 'RAPIDLY'.
(Other definitions for rapidly that I've seen before include "right off the bat" , "With great speed" , "Quickly" , "Fast" , "Speedily" .)