Defeat prior to engagement? Things aren't so good after that (4,6,4)
I believe the answer is:
best before date
'things aren't so good after that' is the definition.
(food isn't as good after its best before date)
'defeat prior to engagement?' is the wordplay.
'defeat' becomes 'best' (besting is a kind of defeating).
'prior to' becomes 'before' (I've seen this before).
'engagement?' becomes 'date' ('date' can be a synonym of 'engagement').
'best'+'before'+'date'='BEST-BEFORE DATE'
(Other definitions for best before date that I've seen before include "Warning to consumer" , "Food freshness marking" , "worse for wear after going out?" , "not so good when taken out?" , "Warning on packet" .)