Notice boring exhibit in darkness (6)
I believe the answer is:
shadow
'darkness' is the definition.
(shadow is a kind of darkness)
'notice boring exhibit' is the wordplay.
'notice' becomes 'ad'.
'boring' is an insertion indicator (some letters must 'bore' a hole into other letters).
'exhibit' becomes 'show' ('show' can be a synonym of 'exhibit').
'ad' put within 'show' is 'SHADOW'.
'in' acts as a link.
(Other definitions for shadow that I've seen before include "Follow closely; pale imitation" , "Secretly tail someone" , "Follow secretly; obscure" , "Darkness caused by obstruction" , "It's cast by something blocking light" .)