Symbol that’s popular holding firm (4)
I believe the answer is:
'symbol' is the definition.
'popular holding firm' is the wordplay.
'popular' becomes 'in'.
'holding' is an insertion indicator.
'firm' becomes 'co' (a firm is a company).
'in' enclosing 'co' is 'ICON'.
'that's' is the link.
(Other definitions for icon that I've seen before include "Holy image or symbol on computer screen" , "A religious painting or representative figure" , "Representation on wooden panel of saint" , "Something revered" , "Adult butterfly" .)