Winding river with grand defence against storms? (4)

I believe the answer is:

mack

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

'defence against storms?' is the definition.
I can't judge whether this definition defines the answer.

'winding river with grand' is the wordplay.
'winding river' becomes 'ack' (I can't explain this - if you can you should believe this answer much more).
'with' says to put letters next to each other.
'grand' becomes 'M' (synonyms).
'ack' after 'm' is 'MACK'.

Can you help me to learn more?

(Other definitions for mack that I've seen before include "- - and Mabel" , "- - the Knife" .)

I've seen this clue in The Independent.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.