Agree to what one might do to unruly hair (6)
I believe the answer is:
permit
'agree to' is the definition.
(thesaurus)
'what one might do to unruly hair' is the wordplay.
I cannot really see how this works, but
'one' could be 'er' (I've seen this in other clues) and 'er' is found within the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for permit that I've seen before include "Allow; make possible" , "Allow - visa" , "Let; enabling document" , "Allow, authorise" , "Licence -- license" .)