Decline and fall from grace with king (4)
I believe the answer is:
'decline' is the definition.
(I know that sink is a more specific form of the action drop)
'fall from grace with king' is the wordplay.
'fall from grace' becomes 'sin' (I've seen this before).
'with' is a charade indicator (letters next to each other).
'king' becomes 'K' (abbreviation for king in chess).
'and' acts as a link.
(Other definitions for sink that I've seen before include "Invest capital in - kitchen basin - founder" , "Submerge; basin" , "Descend into water" , "Go down; wash basin" , "place of corruption" .)