Miserable miss, on reflection, breaking engagement (8)
I believe the answer is:
desolate
'miserable' is the definition.
(thesaurus)
'miss on reflection breaking engagement' is the wordplay.
'miss' becomes 'lose' (I've seen this in other clues).
'on reflection' says the letters should be written in reverse.
'breaking' indicates putting letters inside.
'engagement' becomes 'date' ('date' can be a synonym of 'engagement').
'lose' reversed gives 'esol'.
'esol' inserted within 'date' is 'DESOLATE'.
(Other definitions for desolate that I've seen before include "Bereft" , "Forlorn and unhappy" , "Utterly wretched" , "Wretched, unhappy" , "alone" .)