Dog advert during TV programme (6)
I believe the answer is:
shadow
'dog' is the definition.
(I've seen this before)
'advert during tv programme' is the wordplay.
'advert' becomes 'ad' (synonyms).
'during' is an insertion indicator.
'tv programme' becomes 'show' (a TV show).
'ad' placed inside 'show' is 'SHADOW'.
(Other definitions for shadow that I've seen before include "Dark area, weak remnant (of something)" , "Dark shape (on the Opposition front bench?)" , "Follow; partial darkness" , "Follow - dark patch" , "Unilluminated area" .)