Intimidate key relative (5)
I believe the answer is:
daunt
'intimidate' is the definition.
(I know that daunt is a type of intimidate)
'key relative' is the wordplay.
'key' becomes 'd' (musical key).
'relative' becomes 'aunt' (aunt is a kind of relative).
'd'+'aunt'='DAUNT'
(Other definitions for daunt that I've seen before include "Cause to feel intimidated" , "Make to feel intimidated" , "Dishearten" , "Cow" , "Discourage" .)