No-frills, perhaps? That's cool (9)
I believe the answer is:
unruffled
'cool' is the definition.
(thesaurus)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for unruffled that I've seen before include "Tranquil, unperturbed" , "Not perturbed" , "without frills" , "Calm" , "Not disturbed, remaining even-tempered" .)