Not a healthy basis for walking (5)
I believe the answer is:
plank
'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" .)