Making a cut key is critical (5)
I believe the answer is:
acute
'critical' is the definition.
(similar in meaning)
'making a cut key' is the wordplay.
'making' becomes 'a' (I can't justify this - if you can you should give a lot more credence to this answer).
'key' becomes 'e' (a musical key).
'a'+'cut'+'e'='ACUTE'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for acute that I've seen before include "Keen; severe" , "Sharp-pointed - keenly perceptive" , "Excruciating" , "Urgently pressing" , "Sharp or crucial" .)