Aptly enough, they're involved in the noble art (5)
I believe the answer is:
dukes
'aptly enough they're involved in' is the definition.
The definition suggests an adverb but the answer is not.
'noble art' is the wordplay.
'noble' becomes 'duke' (duke is a kind of noble).
'art' becomes 's' (I am not sure about this - if you are sure you should believe this answer much more).
'duke'+'s'='DUKES'
'the' is the link.
Can you help me to learn more?
(Other definitions for dukes that I've seen before include "Peers of the realm" , "Noblemen" , "eg 21, 22 and Plaza-Toro" , "Titled gents or slangy fists" .)