Wrong king's flag (4)
I believe the answer is:
sink
'flag' is the definition.
(both can mean to tire or fade)
'wrong king's' is the wordplay.
'wrong' becomes 'sin' (wrong can mean a sin or crime).
'king' becomes 'K' (abbreviation for king in chess).
'sin'+'k'='SINK'
(Other definitions for sink that I've seen before include "Drop beneath the surface" , "Scene of iniquity or washing up" , "Basin for washing up etc" , "Droop, submerge" , "Decline - in the kitchen" .)