Not fashionable (6)
I believe the answer is:
uncool
'not fashionable' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for uncool that I've seen before include "Not in fashion or with it" , "hot?" , "Not fashionable (colloq.)" , "Old-fashioned" , "Not with it (but warm?)" .)