One tough youth seen by college (6)
I believe the answer is:
united
'one' is the definition.
(similar in meaning)
'tough youth seen by college' is the wordplay.
'tough youth' becomes 'Ted' (as in the Teddy Boy subculture).
'seen by' is a charade indicator (letters next to each other).
'college' becomes 'uni' (short for university).
'ted' after 'uni' is 'UNITED'.
(Other definitions for united that I've seen before include "Various football teams" , "Affiliated" , "Joined together as in alliance" , "Joined into a single entity" , "See 14" .)