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' (chess abbreviation).
'in' indicates putting letters inside.
'multitude' becomes 'mass' (synonyms).
'k' placed within '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" .)