Possession? Last of men in his power at sea (9)
I believe the answer is:
ownership
'possession?' is the definition.
(ownership is a kind of possession)
'last of men in his power at sea' is the wordplay.
'last of' says to take the final letters.
'in' indicates putting letters inside.
'at sea' is an anagram indicator.
The last letter of 'men' is 'n'.
'his'+'power'='hispower'
'hispower' anagrammed gives 'owership'.
'n' put inside 'owership' is 'OWNERSHIP'.
(Other definitions for ownership that I've seen before include "The right of possessing something" , "Proprietary right of possession" , "Proprietorship, possession" , "Legal title" .)