Old morning test (4)
I believe the answer is:
exam
'test' is the definition.
(I know that test can be written as exam)
'old morning' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'morning' becomes 'am'.
'ex'+'am'='EXAM'
(Other definitions for exam that I've seen before include "'Test, in short (4)'" , "Test (abbrev.)" , "paper" , "Formal knowledge test" , "Students sit it" .)