Bank subsidy about to disappear (8)
I believe the answer is:
gradient
'bank' is the definition.
(thesaurus)
'subsidy about to disappear' is the wordplay.
'subsidy' becomes 'grant' (I've seen this before).
'about' is an insertion indicator.
'to disappear' becomes 'die' (dying is a kind of disappearing).
'grant' placed around 'die' is 'GRADIENT'.
(Other definitions for gradient that I've seen before include "Third OT book" , "Level of steepness" , "Bank" , "Inclination" , "Measure of slope" .)