Changing pants idea is so cool (13)
I believe the answer is:
'cool' is the definition.
'changing pants idea is so' is the wordplay.
'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" , "Rational, impartial" , "Objective, unemotional" , "taking the unbiased view?" .)