One's fitted in gun carriage (4)
I believe the answer is:
gait
'carriage' is the definition.
(I've seen this in another clue)
'one's fitted in gun' is the wordplay.
'one' becomes 'i' (Roman numeral).
'fitted in' is an insertion indicator.
'gun' becomes 'gat' (US informal term for a gun).
'i' put within 'gat' is 'GAIT'.
(Other definitions for gait that I've seen before include "walkway?" , "Style of movement" , "Method of moving" , "Manner of walking or running" , "Walk" .)