Quiet leisurely walk ending in horrendous fiasco (8)
'fiasco' is the definition.
(both can mean a disaster)
'quiet leisurely walk ending in horrendous' is the wordplay.
'quiet' becomes 'sh' (both can be said to tell someone to be quiet).
'leisurely walk' becomes 'amble' (casual or relaxed walk).
'ending in' indicates one should take the final letters.
The final letter of 'horrendous' is 's'.
(Other definitions for shambles that I've seen before include "Chaotic place - slaughterhouse", "Slaughterhouse; mess", "Total mess", "Chaotic state", "Great mess".)