Specimen is small enough (6)
I believe the answer is:
sample
'specimen' is the definition.
(I've seen this before)
'small enough' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'enough' becomes 'ample' (ample means enough or plenty).
's'+'ample'='SAMPLE'
'is' acts as a link.
(Other definitions for sample that I've seen before include "Taste; specimen" , "Small representative piece" , "Drop of" , "Test the specimen" , "instance" .)