Flush and show you've been taken unawares (5)
I believe the answer is:
start
'flush' is the definition.
(thesaurus)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for start that I've seen before include "Outset" , "Begin to recoil?" , "Begin; flinch" , "lead" , "Beginning" .)