Undiluted — cool! (4)
I believe the answer is:
neat
This clue is a double definition.
'undiluted' is the first definition.
(similar in meaning)
'cool' is the second definition.
(synonyms)
(Other definitions for neat that I've seen before include "not mixed with anything else" , "Bovine animal" , "Orderly, uncluttered" , "Smart" , "Trim - tidy - without water" .)