Surrealist drinks one whiskey at the festival (6)
I believe the answer is:
diwali
'the festival' is the definition.
(Hindu festival)
'surrealist drinks one whiskey' is the wordplay.
'surrealist' becomes 'dali' (Salvador Dali).
'drinks' means one lot of letters goes inside another (inserted letters are consumed or drunk).
'one' becomes 'i' (Roman numeral).
'whiskey' becomes 'W' (phonetic alphabet: alpha, bravo, charlie etc.).
'i'+'w'='iw'
'dali' placed around 'iw' is 'DIWALI'.
'at' is the link.
(Other definitions for diwali that I've seen before include "religious festival" , "Festival in honour of Lakshmi" , "Indian festival" , "Light festival" , "Hindu festival of lights" .)