Changing pants idea is so cool (13)
I believe the answer is:
Here is my best explanation:
'cool' is the definition.
'changing pants idea is so' is the subsidiary indication.
'changing' indicates anagramming the letters.
'pantsideaisso' is an anagram of 'dispassionate'.
(Other definitions for dispassionate that I've seen before include "Imperturbable", "Not affected by personal feelings", "Unemotional", "Rational, impartial", "Impartial" and "Objective, unemotional".)