Prospective partner harbouring grudge, ultimately leading to huge waste (9)
I believe the answer is:
devastate
'waste' is the definition.
(I've seen this before)
'prospective partner harbouring grudge ultimately leading to huge' is the wordplay.
'prospective partner' becomes 'date'.
'harbouring' is an insertion indicator.
'ultimately' indicates one should take the final letters.
'leading to' is a charade indicator (letters next to each other) (some letters go before or lead others).
'huge' becomes 'vast' (I've seen this before**).
The last letter of 'grudge' is 'e'.
'e'+'vast'='evast'
'date' enclosing 'evast' is 'DEVASTATE'.
(Other definitions for devastate that I've seen before include "floor" , "Lay waste or totally crush" , "Destroy" , "Spoil" , "Lay waste or overwhelm with grief" .)