It can put a patient out (5)
I believe the answer is:
ether
'it' is the definition.
The definition suggests a singular noun which matches the answer.
'put a patient out' is the wordplay.
'put' becomes 'the' (I've seen this in other clues).
'a patient' becomes 'er' (I can't explain this - if you can you should give a lot more credence to this answer).
'out' means one lot of letters goes inside another.
'the' put into 'er' is 'ETHER'.
'can' is the link.
Can you help me to learn more?
(Other definitions for ether that I've seen before include "Anaesthetic liquid; clear sky" , "Volatile organic liquid once used as a general anaesthetic" , "There goes the anaesthetic" , "I'm volatile" , "Colourless volatile highly inflammable liquid" .)