Small duck's bargains (6)
I believe the answer is:
steals
'bargains' is the definition.
('steal' can be a synonym of 'bargain')
'small duck's' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'duck's' becomes 'teal's' (teal is a type of duck).
's'+'teals'='STEALS'
(Other definitions for steals that I've seen before include "Pilfers" , "Purloins" , "Robs" , "Pilfer, filch" , "Takes without owner's consent" .)