Possibly remain a serviceman (6)
I believe the answer is:
marine
'a serviceman' is the definition.
(marine is a kind of serviceman)
'possibly remain' is the wordplay.
'possibly' indicates anagramming the letters.
'remain' is an anagram of 'MARINE'.
(Other definitions for marine that I've seen before include "seaman" , "Relating to shipping or naval matters" , "Oceanic" , "Concerned with the sea" , "Soldier associated with a navy" .)