King, at the end of the day, is a chess piece (6)
I believe the answer is:
knight
'is a chess piece' is the definition.
(I've seen this before)
'king at the end of the day' is the wordplay.
'king' becomes 'k'.
'at' is a charade indicator (letters next to each other).
'the end of the day' becomes 'night' (I've seen this before).
'k'+'night'='KNIGHT'
(Other definitions for knight that I've seen before include "Give title to; chess piece" , "Ladies' champion" , "rider looking for adventure?" , "Holder of non-hereditary title in UK" , "Chivalrous warrior of olden times" .)