Republican in retail redesigned advert (7)
I believe the answer is:
trailer
'advert' is the definition.
(I know that trailer is a type of advert)
'republican in retail redesigned' is the wordplay.
'republican' becomes 'R' (abbreviation in US politics).
'in' is an insertion indicator.
'redesigned' indicates anagramming the letters.
'retail' anagrammed gives 'tailer'.
'r' put into 'tailer' is 'TRAILER'.
(Other definitions for trailer that I've seen before include "Rear-lit (anag.)" , "Rear section of an articulated lorry" , "one's dragged behind" , "Vehicle designed to be towed by another" , "Not a leading form" .)