Be first to support old flame (5)
I believe the answer is:
exist
'be' is the definition.
('exist' can be a synonym of 'be')
'first to support old flame' is the wordplay.
'first' becomes 'ist'.
'to support' is a charade indicator (letters next to each other) (in a down clue, some letters hold others up).
'old flame' becomes 'ex' (I've seen this in other clues).
'ist' put after 'ex' is 'EXIST'.
(Other definitions for exist that I've seen before include "Subsist" , "Get a life!" , "Be alive" , "quick!" , "Have being or continue living" .)