Maintains a rising pressure (7)
I believe the answer is:
asserts
'maintains' is the definition.
('assert' can be a synonym of 'maintain')
'a rising pressure' is the wordplay.
'rising' shows that the letters should be reversed in order (going up in a down clue).
'pressure' becomes 'stress' (I've seen this before).
'stress' back-to-front is 'sserts'.
'a'+'sserts'='ASSERTS'
(Other definitions for asserts that I've seen before include "States confidently and forcefully" , "States categorically" , "States firmly" , "Declares to be true" .)