Rocks, to a bartender (3)
I believe the answer is:
ice
'rocks to a bartender' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ice that I've seen before include "Diamonds, colloquially" , "If it's thin, skating on it can be dangerous" , "It's cold and slippy" , "Chill - harden" , "Slippery surface" .)