Trace of narcotic in Oriental brew, mouthful that's probably toxic (9)
I believe the answer is:
snakebite
'that's probably toxic' is the definition.
'trace of narcotic in oriental brew mouthful' is the wordplay.
'trace of' says to take the initial letters.
'in' indicates putting letters inside.
'oriental brew' becomes 'sake' (Japanese drink).
'mouthful' becomes 'bite' (bite is a kind of mouthful).
The initial letter of 'narcotic' is 'n'.
'n' inserted within 'sake' is 'snake'.
'snake'+'bite'='SNAKEBITE'
(Other definitions for snakebite that I've seen before include "Drink made of lager and cider" , "Mixture of drinks" , "Cider-lager drink" , "Drink (dangerous if given by 18!)" , "A venomous concoction" .)