Crossword Genius

Not a healthy basis for walking (5)

Ross

I believe the answer is:

plank

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

'walking' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

'not a healthy basis' is the wordplay.
I cannot really understand how this works, but
'a' could be 'an' and 'an' is found within the answer.
This explanation may well be incorrect...

'for' acts as a link.

Can you help me to learn more?

(Other definitions for plank that I've seen before include "Length of wood" , "Prepared piece of wood" , "Building timber" , "Length of sawn timber" , "Wooden board" .)

I've seen this clue in the Evening Standard.
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