Seaman posted missing (6)
I believe the answer is:
absent
'missing' is the definition.
(similar in meaning)
'seaman posted' is the wordplay.
'seaman' becomes 'ab' (short 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 "Not at a specified place" , "Missing, lacking" , "lack of attention" , "Not present - vacuous" , "Not where one should be" .)