Northern valley supplied at last with beer (4)
I believe the answer is:
dale
'northern valley' is the definition.
(I've seen this before)
'supplied at last with beer' is the wordplay.
'at last' suggests the final letters.
'with' is a charade indicator (letters next to each other).
'beer' becomes 'ale' (I've seen this before).
The last letter of 'supplied' is 'd'.
'd'+'ale'='DALE'
(Other definitions for dale that I've seen before include "Low ground" , "Valley; man's name" , "Valley; old radio soap family" , "Lead into poetic valley" , "Up hill and down . . . ." .)