Underage but spotted hanging round the tournament (9)
I believe the answer is:
seventeen
'underage' is the definition.
Both the answer and definition are adjectives. Perhaps you can see an association between them that I can't see?
'spotted hanging round the tournament' is the wordplay.
'spotted' becomes 'seen' (spot can mean to see or notice).
'hanging round' is an insertion indicator.
'the tournament' becomes 'event' (I've seen this before).
'seen' going around 'event' is 'SEVENTEEN'.
'but' acts as a link.
Can you help me to learn more?
(Other definitions for seventeen that I've seen before include "Cardinal number" , "XVII" , "next clue" , "Figure" , "Quite a few" .)