Old morning trial (4)
I believe the answer is:
exam
'trial' is the definition.
(thesaurus)
'old morning' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'morning' becomes 'am' (as in 12:00 AM).
'ex'+'am'='EXAM'
(Other definitions for exam that I've seen before include "Important test" , "Educational test" , "One test" , "Oral maybe" , "Test (abbrev.)" .)