Test old flame by morning (4)
I believe the answer is:
exam
'test' is the definition.
(synonyms)
'old flame by morning' is the wordplay.
'old flame' becomes 'ex' (I've seen this in other clues).
'by' means one lot of letters go next to another.
'morning' becomes 'am'.
'ex'+'am'='EXAM'
(Other definitions for exam that I've seen before include "Test (abbrev.)" , "One taken by candidate" , "One might be taken for English" , "Educational test" , "In brief, a test as in school" .)