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 "It's cast when light is blocked", "Follow; dark area", "Follow closely; pale imitation", "Trace - dog", "Dark area - follow secretly", "Follow secretly; obscure", "Tail", "Hint, remnant; follow secretly", "Inseparable follower" and "Follow closely - dark image - opposition".)