Rock, in rock, paper, scissors (4)
I believe the answer is:
fist
'rock in rock paper scissors' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for fist that I've seen before include "handwriting?" , "Balled hand" , "Hand with the fingers clenched in the palm" , "Hit with a clenched one" , "or be decked in a ring" .)