Dull ship’s officer, one sharing apartment (8)
I believe the answer is:
flatmate
'one sharing apartment' is the definition.
(someone sharing a flat or apartment with another)
'dull ship's officer' is the wordplay.
'dull' becomes 'flat' (synonyms).
'ship's officer' becomes 'mate' (mate is a kind of ship's officer).
'flat'+'mate'='FLATMATE'
(Other definitions for flatmate that I've seen before include "fellow lodger" , "I may share with someone" , "in the rooms one shares" , "for Holmes: Watson?" , "Sharer of accommodation" .)