Allowed to drink when disappointed (3,4)
I believe the answer is:
let down
'disappointed' is the definition.
(I've seen this before)
'allowed to drink' is the wordplay.
'allowed' becomes 'let' (I've seen this before).
'to drink' becomes 'down' (downing is a kind of drinking**).
'let'+'down'='LET-DOWN'
'when' acts as a link.
(Other definitions for let down that I've seen before include "Failed - deflated" , "blow" , "deflating" , "Leave in the lurch" , "Aircraft descent" .)