Missing sailor gets discharged (6)
I believe the answer is:
absent
'missing' is the definition.
(similar in meaning)
'sailor gets discharged' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'gets' is a charade indicator (letters next to each other).
'discharged' becomes 'sent' (I've seen this in another clue).
'ab'+'sent'='ABSENT'
(Other definitions for absent that I've seen before include "Not where one should be" , "elsewhere" , "AWOL" , "Not attending" , "Away" .)