Commercial breaks present gloom (6)
I believe the answer is:
shadow
'gloom' is the definition.
(thesaurus)
'commercial breaks present' is the wordplay.
'commercial' becomes 'ad'.
'breaks' is an insertion indicator.
'present' becomes 'show' ('show' can be a synonym of 'present'**).
'ad' going within 'show' is 'SHADOW'.
(Other definitions for shadow that I've seen before include "Darker part" , "Darkness caused by obstruction" , "Area of darkness caused by obstructing light" , "Partial or complete darkness" , "Dark area from which light is blocked" .)