Good hand (5)
I believe the answer is:
'good hand' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for flush that I've seen before include "Cleanse by running water" , "Rouse game and put to flight" , "Level (with surface); put up (game bird)" , "Pull the chain - rush of emotion - good poker hand - having plenty of money" , "colour" .)