Specimen not originally large enough (5)
I believe the answer is:
ample
'large enough' is the definition.
(I've seen this before)
'specimen not originally' is the wordplay.
'specimen' becomes 'sample' (I've seen this before).
'not originally' means to remove the first letter (the origin of the word is removed).
'sample' with its initial letter taken away is 'AMPLE'.
(Other definitions for ample that I've seen before include "Generous" , "No more, please!" , "Plentiful; capacious" , "Full-figured" , "Spacious" .)