Coin toss choice (5)
I believe the answer is:
tails
'coin toss choice' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tails that I've seen before include "Gambler's choice" , "Coin-tossing call" , "A possible fall of the pound" , "as opposed to heads" , "Ends" .)