Opposed to books being dumped in main road (4)
I believe the answer is:
anti
'opposed to' is the definition.
(I've seen this before)
'books being dumped in main road' is the wordplay.
'books' becomes 'nt' (abbreviation for New Testament).
'being dumped in' means one lot of letters goes inside another.
'main road' becomes 'ai' (resembles A1, major UK road).
'nt' going within 'ai' is 'ANTI'.
(Other definitions for anti that I've seen before include "Who pooh-poohs all" , "Policy's opponent" , "[NO MAN]" , "(Person who is) not in favour" , "'No' voter" .)