First two letters posted off (6)
I believe the answer is:
absent
'off' is the definition.
(thesaurus)
'first two letters posted' is the wordplay.
'first two letters' becomes 'AB' (first two letters of the alphabet).
'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" , "Not attending" , "Not where one should be" , "lack of attention" .)