Direction for traffic remaining behind castle (4,4)

I believe the answer is:

keep left

I'm a little stuck... Click here to teach me more about this clue!

'direction for traffic' is the definition.
I know nothing about this answer so I can't judge whether this works.

'remaining behind castle' is the wordplay.
'remaining' becomes 'left' (synonyms).
'behind' is a charade indicator (letters next to each other).
'castle' becomes 'keep' (keep is a type of castle).
'left' put after 'keep' is 'KEEP LEFT'.

Can you help me to learn more?

Want a hint initially instead of a full solution? Install my app