Number I purchase mostly for flat in E England (9)
I believe the answer is:
ibuprofen
'number' is the definition.
(something that numbs pain)
'i purchase mostly for flat in e england' is the wordplay.
'purchase' becomes 'buy' (buy is a kind of purchase).
'mostly' means to remove the last letter.
'for' becomes 'pro' (as in being 'pro' something).
'flat in e england' becomes 'fen' (fens are flat areas in the East of England).
'buy' with its last letter taken off is 'bu'.
'i'+'bu'+'pro'+'fen'='IBUPROFEN'
(Other definitions for ibuprofen that I've seen before include "Anti-inflammatory medicine" , "Painkiller" , "Drug" , "anaesthetic" .)