Specimen is small and sufficient (6)
I believe the answer is:
sample
'specimen' is the definition.
(I've seen this in another clue)
'small and sufficient' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'and' says to put letters next to each other.
'sufficient' becomes 'ample' (both can mean enough).
's'+'ample'='SAMPLE'
'is' is the link.
(Other definitions for sample that I've seen before include "Specimen, try out" , "Representative portion" , "Small part representative of the whole" , "Cross section" , "Model" .)