Sailor posted away (6)
I believe the answer is:
absent
'away' is the definition.
(similar in meaning)
'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 "elsewhere" , "Away" , "Not where one should be" , "AWOL" , "nowhere to be seen" .)