Former spouse in the morning is trial (4)
I believe the answer is:
exam
'is 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'
(Other definitions for exam that I've seen before include "In brief, a test as in school" , "One taken by candidate" , "One test" , "Students sit it" , "Test (abbrev.)" .)