Stumbling upon clue for 'disconnect' (8)
I believe the answer is:
'disconnect' is the definition.
(I know that uncouple is a type of disconnect)
'stumbling upon clue' is the wordplay.
'stumbling' indicates anagramming the letters.
'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" .)