Mess up, as hair (6)
I believe the answer is:
tousle
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">mess up as hair</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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)" .)