Nymphs in car heading north across main road (7)
I believe the answer is:
naiades
'nymphs' is the definition.
(naiad is a kind of nymph)
'car heading north across main road' is the wordplay.
'car' becomes 'sedan' (sedan is a kind of car).
'heading north' says the letters should be written backwards.
'across' means one lot of letters goes inside another.
'main road' becomes 'AI' (resembles As, major UK road).
'sedan' reversed gives 'nades'.
'nades' going around 'ai' is 'NAIADES'.
'in' is the link.
(Another definition for naiades that I've seen is " nymphs".)