Such a mad rush will make a message unintelligible! (8)
I believe the answer is:
scramble
This clue is a double definition.
'such a mad rush' is the first definition.
(I've seen this before)
'make a message unintelligible' is the second definition.
(I've seen this before)
'will' acts as a link.
(Other definitions for scramble that I've seen before include "make a message unintelligible" , "Jockey for position" , "Distort radio signal" , "render unintelligible ..." , "Clambers (anag) - clamber" .)