Disappointed and allowed to drink in one
I believe the answer is:
let down
'disappointed' is the definition.
(I've seen this before)
'allowed to drink in one' is the wordplay.
'allowed' becomes 'let' (I've seen this before).
'to drink in one' becomes 'down' (to down a drink is to drink it in one go).
'let'+'down'='LET-DOWN'
'and' is the link.
(Other definitions for let down that I've seen before include "blow" , "Aircraft descent" , "Disappoint, betray" , "disillusionment" , "Disappoint, fail to keep promise" .)