Follow notice in exhibition (6)
I believe the answer is:
Here is my best explanation:
'follow' is the definition.
(shadowing is a kind of following)
'notice in exhibition' is the subsidiary indication.
'notice' becomes 'ad'.
'in' indicates putting letters inside.
'exhibition' becomes 'show' (I've seen this in other clues).
'ad' going inside 'show' is 'shadow'.
(Other definitions for shadow that I've seen before include "Secret follower", "Dark image (of opposition?)", "Tail", "It's cast when light is blocked", "Image caused by obscuring of light", "Air of gloom", "Trail", "Follow; partial darkness", "Inseparable follower" and "Hint, remnant; follow secretly".)