Quiz old lover before noon (4)
I believe the answer is:
exam
'quiz' is the definition.
(I've seen this in another clue)
'old lover before noon' is the wordplay.
'old lover' becomes 'ex' (an ex-lover).
'before noon' becomes 'am' (as in AM on a clock).
'ex'+'am'='EXAM'
(Other definitions for exam that I've seen before include "Important test" , "Test (abbr)" , "Something to sit" , "(Academic) test" , "One test" .)