Filthy masses welcoming king (5)
I believe the answer is:
manky
'filthy' is the definition.
(manky means filthy or dirty)
'masses welcoming king' is the wordplay.
'masses' becomes 'many' (many means a lot).
'welcoming' indicates putting letters inside (in sense of 'inviting in').
'king' becomes 'K' (abbreviation for king in chess).
'many' enclosing 'k' is 'MANKY'.
(Other definitions for manky that I've seen before include "Rotten" , "Dirty and unpleasant" .)