Writer with firearm bagging one bird (7)
I believe the answer is:
penguin
'bird' is the definition.
(I know that penguin is a type of flightless bird)
'writer with firearm bagging one' is the wordplay.
'writer with' becomes 'pen' (I've seen this in other clues).
'firearm' becomes 'gun' (I've seen this before).
'bagging' indicates putting letters inside (bag can mean to obtain or contain).
'one' becomes 'i' (Roman numeral).
'gun' going around 'i' is 'guin'.
'pen'+'guin'='PENGUIN'
(Other definitions for penguin that I've seen before include "Flightless marine bird" , "Antarctic seabird with flippers for wings" , "Flightless sea bird" , "Waddling bird" , "Flightless seabird" .)