Rise in subsidy about to stop suddenly (8)
I believe the answer is:
gradient
'rise' is the definition.
(I've seen this before)
'in subsidy about to stop suddenly' is the wordplay.
'in' means one lot of letters goes inside another.
'subsidy' becomes 'grant' (I've seen this before).
'about' shows that the letters should be reversed in order.
'to stop suddenly' becomes 'eid' (I can't justify this - if you can you should give a lot more credence to this answer).
'eid' back-to-front is 'die'.
'grant' enclosing 'die' is 'GRADIENT'.
Can you help me to learn more?
(Other definitions for gradient that I've seen before include "Treading (anag) - slope" , "Angle of slope" , "Level of steepness" , "Rise" , "Degree of slope in a road" .)