Missing seaman dispatched (6)
I believe the answer is:
absent
'missing' is the definition.
(similar in meaning)
'seaman dispatched' is the wordplay.
'seaman' becomes 'ab' (short for able seaman).
'dispatched' becomes 'sent' (I've seen this before).
'ab'+'sent'='ABSENT'
(Other definitions for absent that I've seen before include "nowhere to be seen" , "elsewhere" , "Not attending" , "Missing or dreamy" , "AWOL" .)