One topless specimen is plenty! (5)
I believe the answer is:
ample
'is plenty' is the definition.
(ample means enough or plenty)
'one topless specimen' is the wordplay.
'one topless' means to remove the first letter.
'specimen' becomes 'sample' (I've seen this before).
'sample' with its first letter removed is 'AMPLE'.
(Other definitions for ample that I've seen before include "Generous" , "Plenteous" , "No more, please!" , "Large enough" , "Plentiful; large" .)