Let European into type of party that's celebrated in fancy dress (9)
I believe the answer is:
halloween
'celebrated in fancy dress' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'let european into type of party' is the wordplay.
'let' becomes 'allow' (synonyms).
'european' becomes 'e' (abbreviation e.g. EU).
'into' means one lot of letters goes inside another.
'type of party' becomes 'hen'.
'allow'+'e'='allowe'
'allowe' inserted within 'hen' is 'HALLOWEEN'.
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for halloween that I've seen before include "Night of October thirty-first" , "The eve of All Saints' Day" , "Trick or treat time" , "special night" , "autumn night" .)