Agile small ferret (4)
I believe the answer is:
spry
'agile' is the definition.
(synonyms)
'small ferret' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'ferret' becomes 'pry' (both can mean to seek information).
's'+'pry'='SPRY'
(Other definitions for spry that I've seen before include "Moving quickly and lightly" , "'Active and lively, nimble (4)'" , "Fleet" , "Agile (for one's age?)" , "(Of an old person) lively" .)