Busy road blocked by enormous delivery (6)
I believe the answer is:
doosra
'delivery' is the definition.
(I've seen this before)
'busy road blocked by enormous' is the wordplay.
'busy' indicates anagramming the letters.
'blocked by' indicates putting letters inside (block can mean to plug a gap in something).
'enormous' becomes 'os' (I've seen this in other clues).
'road' with letters rearranged gives 'dora'.
'dora' placed around 'os' is 'DOOSRA'.
(Other definitions for doosra that I've seen before include "Ball that spins" , "special delivery" .)