Mess up, as hair (6)
I believe the answer is:
tousle
'mess up as hair' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tousle that I've seen before include "Treat roughly" , "Dishevel (hair)" , "Make (eg, hair) untidy" , "Make hair untidy by ruffling" , "Ruffle (the hair)" .)