Drink it with starter of toast in Belgium at the end of summer (6)
I believe the answer is:
bitter
'drink' is the definition.
(bitter is a kind of drink)
'it with starter of toast in belgium at the end of summer' is the wordplay.
'with' is a charade indicator (letters next to each other).
'starter of toast' becomes 'te' (I can't justify this - if you can you should believe this answer much more).
'in' is an insertion indicator.
'belgium' becomes 'b' (international vehicle registration code).
'at the end of' suggests the final letters.
The final letter of 'summer' is 'r'.
'it'+'te'='itte'
'b'+'r'='br'
'itte' put inside 'br' is 'BITTER'.
Can you help me to learn more?
(Other definitions for bitter that I've seen before include "Tart like some English beer" , "Acerbic" , "severe" , "Resentful, being piercingly cold" , "Resentful about English beer?" .)