Former lover with sufficient warning (7)
I believe the answer is:
example
'warning' is the definition.
(example is a kind of warning)
'former lover with sufficient' is the wordplay.
'former lover' becomes 'ex'.
'with' says to put letters next to each other.
'sufficient' becomes 'ample' ('ample' is associated in meaning with 'sufficient').
'ex'+'ample'='EXAMPLE'
(Other definitions for example that I've seen before include "Thing illustrating a general rule" , "Illustration or model" , "Paradigm or model" , "Illustrative case" , "Specimen, illustration" .)