Test old lover before noon (4)
I believe the answer is:
exam
'test' is the definition.
(I know that test can be written as exam)
'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 "End-of-course assessment" , "Set of questions evaluating skill or knowledge, in short" , "Oral maybe" , "Something to sit" , "One might be taken for English" .)