Be first to support former partner (5)
I believe the answer is:
exist
'be' is the definition.
(I know that be can be written as exist)
'first to support former partner' is the wordplay.
'first to' becomes 'ist' (I've seen this in other clues).
'support' means one lot of letters go next to another (in a down clue, some letters hold others up).
'former partner' becomes 'ex' (an ex-partner).
'ist' put after 'ex' is 'EXIST'.
(Other definitions for exist that I've seen before include "Live" , "Subsist" , "Have objective reality" , "quick!" , "Get a life!" .)