Crossword Genius

It helps keep the rain off, back in camp (3)

Ross

I believe the answer is:

mac

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

This clue might be a double definition.

'it helps keep the rain off back' is the first definition.
The definition suggests a singular noun which matches the answer.

'camp' is the second definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps there's a link between them I don't understand?

'in' is the link.

Can you help me to learn more?

(Other definitions for mac that I've seen before include "Raincoat (colloq.)" , "It may help you keep dry" , "A Gaelic son, often in name" , "Waterproof raincoat, in short" , "A rainproof" .)

I've seen this clue in the Evening Standard.
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