Trail commercial during performance (6)
I believe the answer is:
shadow
'trail' is the definition.
(I've seen this before)
'commercial during performance' is the wordplay.
'commercial' becomes 'ad'.
'during' is an insertion indicator.
'performance' becomes 'show' (I've seen this before).
'ad' placed into 'show' is 'SHADOW'.
(Other definitions for shadow that I've seen before include "Partial or complete darkness" , "Image caused by obscuring of light" , "Hint, remnant; follow secretly" , "Follow secretly; obscure" , "Inseparable follower" .)