Imposed large deficit possibly involving negotiations at the outset (9)
I believe the answer is:
inflicted
'imposed' is the definition.
('inflict' can be a synonym of 'impose')
'large deficit possibly involving negotiations at the outset' is the wordplay.
'large' becomes 'l'.
'possibly' indicates anagramming the letters.
'involving' indicates putting letters inside.
'at the outset' indicates taking the first letters.
The initial letter of 'negotiations' is 'n'.
'l'+'deficit'='ldeficit'
'ldeficit' is an anagram of 'iflicted'.
'iflicted' going around 'n' is 'INFLICTED'.
(Other definitions for inflicted that I've seen before include "Visited" , "Imposed" .)