Unwashed masses taking king's head (5)
I believe the answer is:
manky
'unwashed' is the definition.
(both can mean dirty)
'masses taking king's head' is the wordplay.
'masses' becomes 'many'.
'taking' indicates putting letters inside.
'head' says to take the initial letters.
The initial letter of 'kings' is 'k'.
'many' placed around 'k' is 'MANKY'.
(Other definitions for manky that I've seen before include "Dirty and unpleasant" , "Rotten" .)