Trial gets old in the morning (4)
I believe the answer is:
exam
'trial' is the definition.
(thesaurus)
'old in the morning' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'in the morning' becomes 'am' (AM on a clock).
'ex'+'am'='EXAM'
'gets' acts as a link.
(Other definitions for exam that I've seen before include "Educational test" , "Important test" , "Students sit it" , "One might be taken for English" , "Formal knowledge test" .)