One's quiet about plan's unhappy outcome (6)
I believe the answer is:
mishap
'unhappy outcome' is the definition.
'one's quiet about plan's' is the wordplay.
'one' becomes 'i' (Roman numeral).
'quiet' becomes 'sh'.
'about' means one lot of letters goes inside another.
'plan' becomes 'map' (I've seen this before).
'i'+'sh'='ish'
'ish' going inside 'map' is 'MISHAP'.
(Other definitions for mishap that I've seen before include "Stroke of bad luck" , "Accident, something going wrong" , "Accident, unlucky happening" , "Contretemps" , "Unlucky development" .)