Test once in the morning (4)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
exam
'test' is the definition.
('exam' can be a synonym of 'test')
'once in the morning' is the wordplay.
'once' becomes 'ex'.
'in the morning' becomes 'am'.
'ex'+'am'='EXAM'
(Other definitions for exam that I've seen before include "Set of questions evaluating skill or knowledge, in short" , "'Test, in short (4)'" , "Students sit it" , "One might be taken for English" , "End-of-course assessment" .)