Critical when abbreviated in notes (5)
I believe the answer is:
acute
'critical' is the definition.
('acute' can be similar in meaning to 'critical')
'abbreviated in notes' is the wordplay.
'abbreviated' becomes 'cut' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'notes' becomes 'ae'.
'cut' going within 'ae' is 'ACUTE'.
'when' acts as a link.
Can you help me to learn more?
(Other definitions for acute that I've seen before include "Shrewd; serious" , "Incisive" , "Sharp or crucial" , "Shrewd; severe" , "Sharp-pointed - keenly perceptive" .)