Giving a name to bird and fish (7)
I believe the answer is:
titling
'giving a name to' is the definition.
(I've seen this in another clue)
'bird and fish' is the wordplay.
'bird' becomes 'tit' (tit is a kind of bird).
'and' says to put letters next to each other.
'fish' becomes 'ling' (ling is a kind of fish).
'tit'+'ling'='TITLING'
(Other definitions for titling that I've seen before include "Giving name to" , "Calling" , "Identifying" .)