Specimen, small, more than adequate (6)
I believe the answer is:
sample
'specimen' is the definition.
(I've seen this before)
'small more than adequate' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'more than adequate' becomes 'ample' (I've seen this before).
's'+'ample'='SAMPLE'
(Other definitions for sample that I've seen before include "Representative portion" , "Take" , "Small representative piece" , "piece to try" , "Cross section" .)