Test old lover in the morning (4)
I believe the answer is:
exam
'test' is the definition.
(I know that test can be written as exam)
'old lover in the morning' is the wordplay.
'old lover' becomes 'ex' (an ex-lover).
'in the morning' becomes 'am' (AM on a clock).
'ex'+'am'='EXAM'
(Other definitions for exam that I've seen before include "Test (abbrev.)" , "Oral maybe" , "One test" , "Scholastic test" , "Important test" .)