Be first to succeed former partner (5)
I believe the answer is:
exist
'be' is the definition.
(synonyms)
'first to succeed former partner' is the wordplay.
'first' becomes 'ist' (ist resembles 1st).
'to succeed' means one lot of letters go next to another.
'former partner' becomes 'ex' (an ex-partner).
'ist' put after 'ex' is 'EXIST'.
(Other definitions for exist that I've seen before include "Subsist" , "Survive" , "Live" , "Have objective reality" , "Scrape by" .)