Old copy can support anaesthetic (4,6)
I believe the answer is:
back number
'old copy' is the definition.
'back number' can be an answer for 'copy' (I have seen 'copy of magazine' mean 'back number' so perhaps 'copy' could also mean 'back number'). I am not sure about the 'old' bit.
'support anaesthetic' is the wordplay.
'support' becomes 'back' (to back is to support or endorse).
'anaesthetic' becomes 'number' (i.e. something that makes things numb).
'back'+'number'='BACK-NUMBER'
'can' is the link.
Can you help me to learn more?
(Other definitions for back number that I've seen before include "what footballers may have on shirts?" , "not a current issue" , "Old magazine" , "An earlier issue" , "orthopaedic anaesthetist?" .)