A bishop posted elsewhere (6)
I believe the answer is:
absent
'elsewhere' is the definition.
(thesaurus)
'a bishop posted' is the wordplay.
'bishop' becomes 'b' (abbreviation used in chess).
'posted' becomes 'sent' ('send' can be a synonym of 'post').
'a'+'b'+'sent'='ABSENT'
(Other definitions for absent that I've seen before include "Not attending" , "nowhere to be seen" , "Missing, lacking" , "Not present - vacuous" , "Missing or dreamy" .)