Specimen, second, more than sufficient (6)
I believe the answer is:
sample
'specimen' is the definition.
(thesaurus)
'second more than sufficient' is the wordplay.
'second' becomes 's' (common abbreviation).
'more than sufficient' becomes 'ample' (I've seen this before).
's'+'ample'='SAMPLE'
(Other definitions for sample that I've seen before include "Take" , "piece to try" , "Model" , "Small representative piece" , "A specimen of maples" .)