Be first to be introduced by former partner (5)
I believe the answer is:
exist
'be' is the definition.
('exist' can be a synonym of 'be')
'first to be introduced by former partner' is the wordplay.
'first' becomes 'ist'.
'to be introduced by' says to put letters next to each other.
'former partner' becomes 'ex'.
'ist' after 'ex' is 'EXIST'.
(Other definitions for exist that I've seen before include "Live" , "Have being or continue living" , "Subsist" , "Survive" , "quick!" .)