Side dish set before king (5)
I believe the answer is:
flank
'side' is the definition.
(I've seen this in another clue)
'dish set before king' is the wordplay.
'dish' becomes 'flan' (I've seen this before).
'set before' means one lot of letters go next to another.
'king' becomes 'K' (abbreviation for king in chess).
'flan'+'k'='FLANK'
(Other definitions for flank that I've seen before include "Side of an animal" , "Haunch" .)