Perhaps post-Brexit tariffs restricting pub, of course, in Western Asia (9)
I believe the answer is:
euphrates
'course in western asia' is the definition.
(river in western Asia)
'perhaps post-brexit tariffs restricting pub' is the wordplay.
I cannot really understand how this works, but
'pub' could be 'ph' (public house) and 'ph' is present in the answer.
This explanation may well be incorrect...
'of' acts as a link.
(Other definitions for euphrates that I've seen before include "River which joins Tigris in Iraq" , "Asian flower" , "River flowing through Turkey, Syria and Iraq" , "River joining the Tigris" , "Asian river" .)