Possibly means a sailor (6)
I believe the answer is:
seaman
'sailor' is the definition.
(I know that seaman is a type of sailor)
'possibly means a' is the wordplay.
'possibly' indicates an anagram.
'means'+'a'='meansa'
'meansa' with letters rearranged gives 'SEAMAN'.
(Other definitions for seaman that I've seen before include "A manse (anag.)" , "Mariner" , "Sailor" , "crew member" .)