Writer with weapon capturing one bird (7)
I believe the answer is:
penguin
'bird' is the definition.
(penguin is a kind of bird)
'writer with weapon capturing one' is the wordplay.
'writer with' becomes 'pen' (I've seen this in other clues).
'weapon' becomes 'gun' (gun is a kind of weapon).
'capturing' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'gun' enclosing 'i' is 'guin'.
'pen'+'guin'='PENGUIN'
(Other definitions for penguin that I've seen before include "King or emperor?" , "Black-and-white antarctic bird" , "Black-and-white Arctic bird" , "Chocolate biscuit" , "Swimming bird" .)