Grasping former partner, wrong to tear into (12)
I believe the answer is:
extortionate
'grasping' is the definition.
(I've seen this before)
'former partner wrong to tear into' is the wordplay.
'former partner' becomes 'ex'.
'wrong' is an anagram indicator.
'to'+'tear'+'into'='totearinto'
'totearinto' is an anagram of 'tortionate'.
'ex'+'tortionate'='EXTORTIONATE'
(Other definitions for extortionate that I've seen before include "Overly demanding" , "Outrageously costly" , "Very pricey" , "(Of a price) much too high" , "Outrageous" .)