Pig eating young fish and bird (7)
I believe the answer is:
sparrow
'bird' is the definition.
(sparrow is a kind of bird)
'pig eating young fish' is the wordplay.
'pig' becomes 'sow' (I've seen this before).
'eating' means one lot of letters goes inside another.
'young fish' becomes 'parr' (parr is a kind of young fish).
'sow' enclosing 'parr' is 'SPARROW'.
'and' is the link.
(Other definitions for sparrow that I've seen before include "Passerine bird" , "Small songbird" , "Small brownish bird" , "Common town bird" , "Little brown bird" .)