... at first expected Biden to welcome British decline (3)
I believe the answer is:
ebb
'decline' is the definition.
(ebb is a kind of decline)
'at first expected biden to welcome british' is the wordplay.
'at first' says to take the initial letters.
'to welcome' is an insertion indicator (in sense of 'inviting in').
'british' becomes 'b' (abbreviation e.g. in 'BBC').
The initial letters of 'expected biden' is 'eb'.
'eb' placed around 'b' is 'EBB'.
(Other definitions for ebb that I've seen before include "state of decline" , "Not to wax" , "(Water) recede" , "Flow back or recede" , "Go out or recede like tide" .)