Writer with firearm bagging one bird (7)
I believe the answer is:
'bird' is the definition.
(penguin is a kind of 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 (I've seen this in other clues).
'one' becomes 'i' (Roman numeral).
'gun' placed around 'i' is 'guin'.
(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" .)