On reflection, it never keeps happening (5)
I believe the answer is:
event
'happening' is the definition.
(thesaurus)
'on reflection it never keeps' is the wordplay.
'on reflection' says the letters should be written in reverse.
'keeps' indicates a hidden word.
'tneve' is hidden within 'it never'.
'tneve' back-to-front is 'EVENT'.
(Other definitions for event that I've seen before include "contest in programme" , "heat, perhaps" , "Result; incident" , "pole vault maybe" , "Meeting" .)