Key is faulty I'm afraid (5)
I believe the answer is:
yikes
'i'm afraid' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.
'key is faulty' is the wordplay.
'faulty' is an anagram indicator.
'key'+'is'='keyis'
'keyis' anagrammed gives 'YIKES'.
Can you help me to learn more?
(Other definitions for yikes that I've seen before include "Watch out" , "that was hot!" , "Exclamation of surprise or alarm" , "Expression of alarm or astonishment" , "by George" .)