First of them comment having taken in ad's distinguishing characteristic (9)
I believe the answer is:
trademark
'distinguishing characteristic' is the definition.
(a distinctive feature or quality)
'first of them comment having taken in ad's' is the wordplay.
'first of them' becomes 't' (rst letter of 'them').
'comment' becomes 'remark' (synonyms).
'having taken in' is an insertion indicator.
'remark' placed around 'ad' is 'rademark'.
't'+'rademark'='TRADEMARK'
(Other definitions for trademark that I've seen before include "Proprietary name" , "sign of concern" , "Distinguishing feature" , "Registered sign of a manufacturer" , "Company's unique symbol" .)