Rough element reportedly criminal, in the main (7)
I believe the answer is:
corsair
'rough' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'element reportedly criminal in the main' is the wordplay.
'element' becomes 'core' (I have seen 'essential element' mean 'core' so perhaps 'element' could also mean 'core').
'reportedly' indicates a 'sounds like' (homophone) clue.
'criminal in the main' becomes 'sair' (I can't justify this - if you can you should believe this answer much more).
'core' is a homophone of 'cor'.
'cor'+'sair'='CORSAIR'
Can you help me to learn more?
(Other definitions for corsair that I've seen before include "A swift pirate ship" , "Barbary pirate - pirate ship" , "20 across" , "Pirate from N. Africa" , "Main villain" .)