During display, notice dog (6)
I believe the answer is:
shadow
'dog' is the definition.
(thesaurus)
'during display notice' is the wordplay.
'during' indicates putting letters inside.
'display' becomes 'show' ('show' can be a synonym of 'display').
'notice' becomes 'ad'.
'show' placed around 'ad' is 'SHADOW'.
(Other definitions for shadow that I've seen before include "It's cast when light is blocked" , "Unilluminated area" , "opposing counterpart" , "Dark area, weak remnant (of something)" , "Hint, remnant; follow secretly" .)