Terrible to put key on edge (4)
I believe the answer is:
grim
'terrible' is the definition.
(grim can mean terrible or horrible)
'key on edge' is the wordplay.
'key' becomes 'g' (musical key).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'edge' becomes 'rim' (rim is a kind of edge).
'g'+'rim'='GRIM'
'to put' is the link.
(Other definitions for grim that I've seen before include "Dismal, dour" , "Formidably stern" , "(Of humour) black; unappealing, depressing" , "Resolute" , "Stern, cruel" .)