Former spouse in the morning is a trial (4)
I believe the answer is:
exam
'trial' is the definition.
(thesaurus)
'former spouse in the morning' is the wordplay.
'former spouse' becomes 'ex' (an ex-spouse).
'in the morning' becomes 'am' (AM on a clock).
'ex'+'am'='EXAM'
'is a' acts as a link.
(Other definitions for exam that I've seen before include "End-of-course assessment" , "One might be taken for English" , "One taken by candidate" , "Test (abbrev.)" , "Important test" .)