Die in act, upsetting show (8)
I believe the answer is:
indicate
'show' is the definition.
('indicate' can be a synonym of 'show')
'die in act upsetting' is the wordplay.
'upsetting' indicates an anagram.
'die'+'in'+'act'='dieinact'
'dieinact' anagrammed gives 'INDICATE'.
(Other definitions for indicate that I've seen before include "Point, show, signal" , "(Give a) signal" , "Point out or give some notion of" , "Point out, suggest" , "Mark" .)