Small enough specimen (6)
I believe the answer is:
sample
'specimen' is the definition.
(thesaurus)
'small enough' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'enough' becomes 'ample' (I've seen this before).
's'+'ample'='SAMPLE'
(Other definitions for sample that I've seen before include "Model" , "Small part representing the whole" , "Experience" , "Small part intended to represent the whole" , "Small representative piece" .)