Deceptive appearances? Everyone in fancy dress disheartened (7)
I believe the answer is:
fallacy
'deceptive appearances? everyone' is the definition.
I can't tell whether this defines the answer.
'in fancy dress disheartened' is the wordplay.
'in fancy' becomes 'falla' (I can't justify this - if you can you should believe this answer much more).
'dress' becomes 'curry' (synonyms).
'disheartened' means to remove the middle letters (the middle or heart is removed).
'curry' with its centre removed is 'cy'.
'falla'+'cy'='FALLACY'
Can you help me to learn more?
(Other definitions for fallacy that I've seen before include "A mistake in reasoning" , "Delusion" , "Illogical argument" , "Faulty reasoning or mistaken belief" , "Incorrect notion based on false reasoning" .)