Stumbling upon clue for 'disconnect' (8)
I believe the answer is:
uncouple
'disconnect' is the definition.
(I know that uncouple is a type of disconnect)
'stumbling upon clue' is the wordplay.
'stumbling' indicates anagramming the letters.
'upon'+'clue'='uponclue'
'uponclue' anagrammed gives 'UNCOUPLE'.
'for' is the link.
(Other definitions for uncouple that I've seen before include "Release" , "Detach (truck from train)" , "Separate (carriage from train)" , "Disconnect" , "Detach one train carriage from another" .)