A bishop, dispatched, is missing (6)
I believe the answer is:
absent
'is missing' is the definition.
(similar in meaning)
'a bishop dispatched' is the wordplay.
'bishop' becomes 'B' (abbreviation for bishop in chess).
'dispatched' becomes 'sent' (I've seen this in another clue).
'a'+'b'+'sent'='ABSENT'
(Other definitions for absent that I've seen before include "Missing, lacking" , "Not attending" , "Forgetful, with mind on other things" , "nowhere to be seen" , "Missing or dreamy" .)