It allows you to do something to fix unruly hair (6)
I believe the answer is:
permit
'it allows you to do' is the definition.
The definition suggests a singular noun which matches the answer.
'something to fix unruly hair' is the wordplay.
I cannot really understand how this works, but
'something' could be 'it' (I've seen this in other clues) and 'it' is found within the answer.
'hair' could be 'perm' (perm is a kind of hair) and 'perm' is located in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for permit that I've seen before include "Allow; make possible" , "Allow, sanction" , "Consent to, allow" , "Allow - visa" , "Official document giving authorisation" .)