Rearrest man I'm fighting -- he might be at sea (6-7)
I believe the answer is:
master-mariner
'he might be at sea' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'rearrest man i'm fighting' is the wordplay.
'fighting' indicates an anagram (I've seen this in other clues).
'rearrest'+'man'+'im'='rearrestmanim'
'rearrestmanim' with letters rearranged gives 'MASTER-MARINER'.
Can you help me to learn more?
(Other definitions for master-mariner that I've seen before include "Person qualified to be a ship's captain" , "Captain of merchantman" , "Captain of merchant vessel" , "Seaman - marries RN mate! (anag)" .)