Brief Encounter? At end, nothing's changed (3-5,5)
I believe the answer is:
one-night stand
'brief encounter?' is the definition.
'at end nothing's changed' is the wordplay.
'changed' indicates an anagram.
'at'+'end'+'nothings'='atendnothings'
'atendnothings' anagrammed gives 'ONE-NIGHT STAND'.
(Other definitions for one-night stand that I've seen before include "Very brief affair" , "Very brief fling" , "Not a lasting affair" , "Singularly brief physical relationship" , "Brief amour" .)