Drunken English occupying poor Britain leave at last (9)
I believe the answer is:
inebriate
'drunken' is the definition.
('inebriate' can be a synonym of 'drunken')
'english occupying poor britain leave at last' is the wordplay.
'english' becomes 'e' (abbreviation).
'occupying' is an insertion indicator.
'poor' is an anagram indicator.
'at last' indicates one should take the final letters.
The final letter of 'leave' is 'e'.
'britain'+'e'='britaine'
'britaine' with letters rearranged gives 'inbriate'.
'e' inserted inside 'inbriate' is 'INEBRIATE'.
(Other definitions for inebriate that I've seen before include "Being loaded" , "Exhilarate" , "boozer" , "Habitual drinker" , "Get drunk" .)