Sailor posted as 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'
'as' acts as a link.
(Other definitions for absent that I've seen before include "AWOL" , "Inattentive" , "Away" , "Missing, lacking" , "lack of attention" .)