Test former partner before noon (4)
I believe the answer is:
'test' is the definition.
'former partner before noon' is the wordplay.
'former partner' becomes 'ex' (an ex-partner).
'before noon' becomes 'am'.
(Other definitions for exam that I've seen before include "One taken by candidate" , "School test (colloq.)" , "Formal knowledge test" , "'Test, in short (4)'" , "paper" .)