Old lover to mention thrill (6)
I believe the answer is:
excite
'thrill' is the definition.
(thrill can mean to excite)
'old lover to mention' is the wordplay.
'old lover' becomes 'ex' (an ex-lover).
'to mention' becomes 'cite' (to cite is to mention or quote).
'ex'+'cite'='EXCITE'
(Other definitions for excite that I've seen before include "Send" , "Cause strong feelings in" , "Get worked up" , "Stir emotionally" , "Act as a stimulant" .)