Reason for cancelling outdoor event? Minor star is upset (9)
I believe the answer is:
rainstorm
'reason for cancelling outdoor event?' is the definition.
(an outdoor event might cancelled due to rain)
'minor star is upset' is the wordplay.
'is upset' indicates an anagram (I've seen 'are upset' mean this).
'minor'+'star'='minorstar'
'minorstar' with letters rearranged gives 'RAINSTORM'.
(Other definitions for rainstorm that I've seen before include "Meteorological phenomenon" , "In bad fall" , "Heavy downpour" , "Heavy fall" , "squall" .)