Walking exercise within the (American) law (14)
I believe the answer is:
constitutional
'walking exercise' is the definition.
'constitutional' can be an answer for 'exercise' (I've seen this before). I am not sure about the 'walking' bit.
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for constitutional that I've seen before include "Principled" , "A walk taken for its benefit to health" , "steps taken to improve health" , "authorised by the body politic" , "within the law" .)