Formal custom sounds correct (4)
I believe the answer is:
'formal custom' is the definition.
'rite' can be an answer for 'custom' (rite is a kind of custom). I'm not sure about the 'formal' bit.
'sounds correct' is the wordplay.
'sounds' indicates a 'sounds like' (homophone) clue.
'correct' becomes 'right' (synonyms).
'right' is a homophone of 'RITE'.
Can you help me to learn more?
(Other definitions for rite that I've seen before include "Formal procedure" , "Tries awkwardly to perform rituals" , "(Words for) ceremony" , "form of service" , "Religious ritual" .)