Sun is authentic, terribly cool (14)
I believe the answer is:
unenthusiastic
'cool' is the definition.
(I've seen this before)
'sun is authentic terribly' is the wordplay.
'terribly' is an anagram indicator.
'sun'+'is'+'authentic'='sunisauthentic'
'sunisauthentic' with letters rearranged gives 'UNENTHUSIASTIC'.
(Other definitions for unenthusiastic that I've seen before include "Not keen" , "Cool" , "Showing no interest" , "apathetic" , "like a fan? No" .)