Poach small duck (5)
I believe the answer is:
steal
'poach' is the definition.
(thesaurus)
'small duck' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'duck' becomes 'teal' (teal is a kind of duck).
's'+'teal'='STEAL'
(Other definitions for steal that I've seen before include "Filch" , "Pinch" , "Shoplift" , "a snip!" , "Lift - pocket" .)