Head leaving specimen — more than enough (5)
I believe the answer is:
ample
'more than enough' is the definition.
(I've seen this before)
'head leaving specimen' is the wordplay.
'head leaving' means to remove the first letter.
'specimen' becomes 'sample' (I've seen this before).
'sample' with its first letter taken away is 'AMPLE'.
(Other definitions for ample that I've seen before include "Large enough" , "Easily sufficient" , "Plenteous" , "Big" , "Buxom" .)