Pass instruction on how to make hair curl (6)
I believe the answer is:
permit
'pass' is the definition.
(both can mean to allow)
'hair curl' is the wordplay.
'hair' becomes 'perm' (perm is a kind of hair).
'curl' becomes 'it' (I can't justify this - if you can you should believe this answer much more).
'perm'+'it'='PERMIT'
'instruction on how to make' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
(Other definitions for permit that I've seen before include "Allow, authorise" , "Let; enabling document" , "Consent to, allow" , "Official document giving authorisation" , "Let - pass" .)