Test former spouse in the morning (4)
I believe the answer is:
exam
'test' is the definition.
(I know that test can be written as exam)
'former spouse in the morning' is the wordplay.
'former spouse' becomes 'ex' (an ex-spouse).
'in the morning' becomes 'am' (short for ante meridiem - before noon).
'ex'+'am'='EXAM'
(Other definitions for exam that I've seen before include "Written test" , "End-of-course assessment" , "Set of questions evaluating skill or knowledge, in short" , "Scholastic test" , "Educational test" .)