Hemmed in by crowd, king hides (5)
I believe the answer is:
masks
'hides' is the definition.
(I've seen this in another clue)
'hemmed in by crowd king' is the wordplay.
'hemmed in by' means one lot of letters goes inside another.
'crowd' becomes 'mass' (massing is a kind of crowding).
'king' becomes 'K' (abbreviation for king in chess).
'mass' placed around 'k' is 'MASKS'.
(Other definitions for masks that I've seen before include "Coverings to conceal the face" , "Hides, disguises" .)