Met geisha, slyly, at college? You've been found out! (3,4,2,2)
I believe the answer is:
the game is up
'you've been found out' is the definition.
(phrase said when someone has been caught)
'met geisha slyly at college?' is the wordplay.
'slyly' indicates anagramming the letters.
'at college?' becomes 'up'.
'met'+'geisha'='metgeisha'
'metgeisha' anagrammed gives 'thegameis'.
'thegameis'+'up'='THE GAME IS UP'
(Other definitions for the game is up that I've seen before include "Springer's been active" , "The scheme has failed, all is revealed" , "One's crime is revealed" , "The deception's revealed" , "The beaters have done their work" .)