Hep, as cats (4)
I believe the answer is:
cool
'hep as cats' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for cool that I've seen before include "Fashionable (but don't lose yours!)" , "Super!" , "Chilled" , "Calm; chilly" , "Lose heat" .)