Quash underground resistance on return (5)
I believe the answer is:
rebut
'quash' is the definition.
(I've seen this before)
'underground resistance on return' is the wordplay.
'underground' becomes 'tube' (the London Underground is nicknamed the Tube).
'resistance' becomes 'r' (abbreviation in physics and electronics).
'on return' says the letters should be written backwards.
'tube'+'r'='tuber'
'tuber' backwards is 'REBUT'.
(Other definitions for rebut that I've seen before include "Contradict or disprove" , "Overthrow by argument or proof" , "Prove the claim wrong" , "Claim or prove to be false" , "Prove (something) to be false" .)