Small and large specimen (6)
I believe the answer is:
sample
'specimen' is the definition.
(thesaurus)
'small and large' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'and' is a charade indicator (letters next to each other).
'large' becomes 'ample' ('ample' can be similar in meaning to 'large').
's'+'ample'='SAMPLE'
(Other definitions for sample that I've seen before include "A specimen of maples" , "Cross section" , "Specimen; sound clip" , "Experience" , "Taste; specimen" .)