Rudely, take fish while they're fighting (9)
I believe the answer is:
wrestling
'while they're fighting' is the definition.
The definition suggests an adverb but the answer is not.
'rudely take fish' is the wordplay.
'rudely' becomes 'wrest' (I can't justify this - if you can you should believe this answer much more).
'take' means one lot of letters go next to another.
'fish' becomes 'ling' (ling is a kind of fish).
'wrest'+'ling'='WRESTLING'
Can you help me to learn more?
(Other definitions for wrestling that I've seen before include "Hand-to-hand combat" , "event" , "Hand-to-hand grappling" , "gripping contest" , "Ring fighting" .)