They disguise king in multitude (5)
I believe the answer is:
masks
'they disguise' is the definition.
Although both the answer and 'disguise' are verbs, they are not in the same form of the verb.
'king in multitude' is the wordplay.
'king' becomes 'K' (abbreviation for king in chess).
'in' means one lot of letters goes inside another.
'multitude' becomes 'mass' (synonyms).
'k' put into 'mass' is 'MASKS'.
Can you help me to learn more?
(Other definitions for masks that I've seen before include "Coverings to conceal the face" , "Hides, disguises" .)