Unwashed masses to surround king (5)
I believe the answer is:
manky
'unwashed' is the definition.
(both can mean dirty)
'masses to surround king' is the wordplay.
'masses' becomes 'many' (many means a lot).
'to surround' means one lot of letters goes inside another.
'king' becomes 'K' (abbreviation for king in chess).
'many' going around 'k' is 'MANKY'.
(Other definitions for manky that I've seen before include "Rotten" , "Dirty and unpleasant" .)