Disprove underground river’s rising (5)
I believe the answer is:
rebut
'disprove' is the definition.
(rebutting is a kind of disproving)
'underground river's rising' is the wordplay.
'underground' becomes 'tube' ('tube' can be a synonym of 'underground').
'river' becomes 'r'.
'rising' is a reversal indicator (going up in a down clue).
'tube'+'r'='tuber'
'tuber' back-to-front is 'REBUT'.
(Other definitions for rebut that I've seen before include "Give the lie to" , "Brute can refute" , "Scotch" , "Refute - disprove" , "Disprove an accusation" .)