Changing pants idea is so cool (13)
I believe the answer is:
dispassionate
'cool' is the definition.
(thesaurus)
'changing pants idea is so' is the wordplay.
'changing' indicates anagramming the letters.
'pants'+'idea'+'is'+'so'='pantsideaisso'
'pantsideaisso' is an anagram of 'DISPASSIONATE'.
(Other definitions for dispassionate that I've seen before include "Imperturbable" , "Not affected by personal feelings" , "Rational, impartial" , "Objective, unemotional" , "taking the unbiased view?" .)