Express carries publicity for follower (6)
I believe the answer is:
shadow
'follower' is the definition.
(shadow is a kind of follower)
'express carries publicity' is the wordplay.
'express' becomes 'show' (synonyms).
'carries' is an insertion indicator.
'publicity' becomes 'ad' (advert).
'show' going around 'ad' is 'SHADOW'.
'for' is the link.
(Other definitions for shadow that I've seen before include "Inseparable follower" , "Dark area - follow secretly" , "Follow closely; pale imitation" , "Secretly tail someone" , "Partial or complete darkness" .)