Most rowdy sporting tunes include game by line one (9)
I believe the answer is:
Here is my best explanation:
'most rowdy' is the definition.
'sporting tunes include game by line one' is the wordplay.
'sporting' indicates anagramming the letters.
'include' means one lot of letters goes inside another.
'game' becomes 'ru' (rugby union).
'by line one' becomes 'li' (I can't explain this - if you can you should give a lot more credence to this answer).
'tunes' anagrammed gives 'unest'.
'unest' placed around 'ruli' is 'UNRULIEST'.
(Another definition for unruliest that I've seen is "Most disobedient".)