Admit academic robe's no good at college (3,2)
I believe the answer is:
own up
'admit' is the definition.
(I've seen this before)
'academic robe's no good at college' is the wordplay.
'academic robe' becomes 'gown' (I've seen this before).
'no' is a deletion indicator.
'good' becomes 'g' (abbreviation).
'at college' becomes 'up'.
'gown' with 'g' taken away is 'own'.
'own'+'up'='OWN UP'
(Other definitions for own up that I've seen before include "Confess, admit guilt (3.2)" , "Confess, acknowledge fault" , "Admit to wrongdoing" , "Come clean (about wrongdoing)" , "Accept responsibility" .)