Boyfriend is guy Manon treated roughly (5,3)
I believe the answer is:
young man
'boyfriend' is the definition.
('young man' can mean someone's boyfriend)
'guy manon treated roughly' is the wordplay.
'treated roughly' indicates anagramming the letters (I've seen this in other clues).
'guy'+'manon'='guymanon'
'guymanon' is an anagram of 'YOUNG MAN'.
'is' acts as a link.
(Other definitions for young man that I've seen before include "Youth" , "sweetheart" , "Boyfriend - teenager" , "Old codger? Quite the opposite" .)