Be first to be sat on by old flame (5)
I believe the answer is:
exist
'be' is the definition.
(synonyms)
'first to be sat on by old flame' is the wordplay.
'first to' becomes 'ist' (I've seen this in other clues).
'be sat on by' says to put letters next to each other.
'old flame' becomes 'ex' (an ex-partner).
'ist' put after 'ex' is 'EXIST'.
(Other definitions for exist that I've seen before include "Scrape by" , "Subsist" , "Survive" , "quick!" , "Get a life!" .)