Be first to support old lover (5)
I believe the answer is:
exist
'be' is the definition.
('exist' can be a synonym of 'be')
'first to support old lover' 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 lover' becomes 'ex' (I've seen this in other clues).
'ist' after 'ex' is 'EXIST'.
(Other definitions for exist that I've seen before include "Have being or continue living" , "Live" , "Be alive" , "quick!" , "Survive" .)