Bank permit about to expire (8)
I believe the answer is:
gradient
'bank' is the definition.
(both can mean sloped ground)
'permit about to expire' is the wordplay.
'permit' becomes 'grant' (I've seen this before).
'about' means one lot of letters goes inside another.
'to expire' becomes 'die' ('die' can be a synonym of 'expire').
'grant' placed around 'die' is 'GRADIENT'.
(Other definitions for gradient that I've seen before include "Perhaps one in ten" , "Third OT book" , "Degree of slope in a road" , "Bank" , "Treading (anag) - slope" .)