Scowl from volunteer missing event (4)
I believe the answer is:
lour
'scowl' is the definition.
(thesaurus)
'from volunteer missing event' is the wordplay.
'from' is an anagram indicator (from the letters).
'missing' indicates named letters should be taken away.
'volunteer' with 'event' taken away is 'olur'.
'olur' is an anagram of 'LOUR'.
(Other definitions for lour that I've seen before include "Threatening appearance" , "(Of sky) look dark" , "; Scowl (of the sky) look threatening" , "Appear sullen or threatening" , "Look sullen - frown" .)