Vessel carrying indisposed males, ones in the firing line! (12)
I believe the answer is:
artillerymen
'ones in the firing line' is the definition.
(people in the firing line during a battle)
'vessel carrying indisposed males' is the wordplay.
'vessel' becomes 'artery' (artery is a kind of vessel).
'carrying' means one lot of letters goes inside another.
'indisposed' becomes 'ill' (similar in meaning).
'males' becomes 'men' (man is a kind of male).
'artery' placed around 'ill' is 'artillery'.
'artillery'+'men'='ARTILLERYMEN'
(Another definition for artillerymen that I've seen is " Gunnery soldiers".)