Muss up (6)
I believe the answer is:
tousle
'muss up' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tousle that I've seen before include "Mess around" , "Ruffle (the hair)" , "disarray" , "Dishevel (hair)" , "Ruffle - disarrange" .)