Both comrades at sea out of action (4,2,6)
I believe the answer is:
hors de combat
'out of action' is the definition.
('hors de combat' can be a synonym of 'out of action')
'both comrades at sea' is the wordplay.
'at sea' indicates an anagram.
'both'+'comrades'='bothcomrades'
'bothcomrades' with letters rearranged gives 'HORS DE COMBAT'.
(Other definitions for hors de combat that I've seen before include "Disabled, out of the fight" , "Injured and out of action (French)" , "Out of action due to injury" .)