Fumble for cord on key (5)
I believe the answer is:
grope
'fumble' is the definition.
(I've seen this before)
'cord on key' is the wordplay.
'cord' becomes 'rope' (rope is a kind of cord).
'on' is a charade indicator (letters next to each other).
'key' becomes 'g' (I've seen this in other clues**).
'rope' after 'g' is 'GROPE'.
'for' acts as a link.
(Other definitions for grope that I've seen before include "Search blindly" , "Feel uncertainly along" , "Feel about as in the dark" , "Paw" , "Fumble" .)