After one mile heavy walk start to experience collapse (7)
I believe the answer is:
implode
'collapse' is the definition.
(imploding is a kind of collapsing)
'after one mile heavy walk start to experience' is the wordplay.
'after' says to put letters next to each other.
'one' becomes 'i' (Roman numeral).
'mile' becomes 'm' (common abbreviation).
'heavy walk' becomes 'plod' (plodding is a kind of walking. I am not sure about the 'heavy' bit.).
'start to' indicates taking the first letters.
The initial letter of 'experience' is 'e'.
'i' put next to 'm' is 'im'.
'im'+'plod'+'e'='IMPLODE'
Can you help me to learn more?
(Other definitions for implode that I've seen before include "Violently collapse inwards" , "Cause to burst inwards" , "Collapse suddenly inwards" , "Burst inward" .)