British leaving fundamental ingredient unaltered (2,2)
I believe the answer is:
as is
'unaltered' is the definition.
(ie something left 'as is')
'british leaving fundamental ingredient' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in BBC).
'leaving' suggests deleting specific letters.
'fundamental ingredient' becomes 'basis' (I've seen this in another clue).
'basis' with 'b' removed is 'AS IS'.
(Other definitions for as is that I've seen before include "in the present circumstances" , "In the existing circumstances" , "not changed" , "current state" , "In present condition" .)