Right to block postponement? Get lost (5)
I believe the answer is:
stray
'lost' is the definition.
(thesaurus)
'right to block postponement?' is the wordplay.
'right' becomes 'r' (common abbreviation).
'to block' is an insertion indicator (block can mean to fill a hole in something).
'postponement?' becomes 'stay' (I've seen this before).
'r' put into 'stay' is 'STRAY'.
'get' acts as a link.
(Other definitions for stray that I've seen before include "out of place" , "Wander from the path" , "digress" , "Depart from the right path" , "Waif and - -" .)