A cut key is sharp (5)
I believe the answer is:
acute
'sharp' is the definition.
(I know that sharp can be written as acute)
'a cut key' is the wordplay.
'key' becomes 'e' (a musical key).
'a'+'cut'+'e'='ACUTE'
'is' is the link.
(Other definitions for acute that I've seen before include "(Of an angle) less than 90deg" , "Extremely sharp or intense" , "Quick-witted" , "Shrewd; serious" , "Penetrating; accent" .)