Follow poster used in display (6)
I believe the answer is:
shadow
'follow' is the definition.
(shadowing is a kind of following)
'poster used in display' is the wordplay.
'poster' becomes 'ad'.
'used in' means one lot of letters goes inside another.
'display' becomes 'show' (synonyms).
'ad' placed inside 'show' is 'SHADOW'.
(Other definitions for shadow that I've seen before include "Darker part" , "Trail" , "Image cast - dog" , "Sort of boxer - dog" , "Dark area - referring to parliamentary opposition" .)