Trio going round at noon are menacing (8)
I believe the answer is:
threaten
'are menacing' is the definition.
(I've seen this before)
'trio going round at noon' is the wordplay.
'trio' becomes 'three' (synonyms).
'going round' is an insertion indicator (some letters go around others).
'noon' becomes 'n'.
'three' enclosing 'at' is 'threate'.
'threate'+'n'='THREATEN'
(Other definitions for threaten that I've seen before include "Put in jeopardy" , "Menace - impend" , "'Menace, intimidate (8)'" , "State an intention to harm" , "Put at risk, endanger" .)