Sailor posted missing (6)
I believe the answer is:
absent
'missing' is the definition.
('absent' can be similar in meaning to 'missing')
'sailor posted' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'posted' becomes 'sent' ('send' can be a synonym of 'post').
'ab'+'sent'='ABSENT'
(Other definitions for absent that I've seen before include "nowhere to be seen" , "Inattentive" , "Not present or focused" , "Away" , "Missing or dreamy" .)