A duck eating a catch (4)
'a catch' is the definition.
'a duck eating' is the wordplay.
'duck' becomes 'nil' (I've seen this before).
'eating' is an insertion indicator.
'a' put into 'nil' is 'NAIL'.
(Other definitions for nail that I've seen before include "Hard plate at end of digit", "Fastening device", "Apprehend", "Metal spike with head", "Hit this on the head to be dead right".)