Old but cool (5)
I believe the answer is:
retro
'old but cool' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for retro that I've seen before include "Old-fashioned style" , "In a backward-looking style" , "Imitating a bygone style" , "Prefix for old-style" , "Imitative of an older style" .)