Handsome youth is following American college fellow (6)
I believe the answer is:
Here is my best explanation:
'handsome youth' is the definition.
(I've seen this before)
'is following american college fellow' is the subsidiary indication.
'following' says to put letters next to each other.
'american college' becomes 'a' (I can't explain this - if you can you should give a lot more credence to this answer).
'fellow' becomes 'don' (I've seen this before).
'is' after 'adon' is 'adonis'.
(Other definitions for adonis that I've seen before include "Extremely attractive young man from Greek mythology", "Very handsome man", "Handsome Greek loved by Aphrodite", "Said no to the Greek youth", "Legendary lover", "dish", "Youth loved by Aphrodite", "Greek idol", "In Greek myth, handsome youth beloved of Aphrodite" and "Handsome chap".)