Popular ship’s officer, one serving time, perhaps (6)
I believe the answer is:
inmate
'one serving time perhaps' is the definition.
(someone serving time in prison)
'popular ship's officer' is the wordplay.
'popular' becomes 'in'.
'ship's officer' becomes 'mate' (mate is a kind of ship's officer).
'in'+'mate'='INMATE'
(Other definitions for inmate that I've seen before include "you mightn't want him to escape" , "Cell resident" , "Inhabitant of prison or other institution" , "One living in institution" , "One living in (prison)" .)