Naming bird and fish (7)
I believe the answer is:
titling
'naming' is the definition.
(titling is a kind of naming)
'bird and fish' is the wordplay.
'bird' becomes 'tit' (tit is a kind of bird).
'and' means one lot of letters go next to another.
'fish' becomes 'ling' (ling is a kind of fish).
'tit'+'ling'='TITLING'
(Other definitions for titling that I've seen before include "Calling" , "Identifying" , "Giving name to" .)