Side dish set before king (5)
I believe the answer is:
flank
'side' is the definition.
(I've seen this before)
'dish set before king' is the wordplay.
'dish' becomes 'flan' (I've seen this before).
'set before' is a charade indicator (letters next to each other).
'king' becomes 'K' (chess abbreviation).
'flan'+'k'='FLANK'
(Other definitions for flank that I've seen before include "Side of an animal" , "Haunch" .)