Trio snatching at Scots hooligan were menacing (10)
I believe the answer is:
threatened
'were menacing' is the definition.
(to threaten is to behave menacingly)
'trio snatching at scots hooligan' is the wordplay.
'trio' becomes 'three' (synonyms).
'snatching' means one lot of letters goes inside another.
'scots hooligan' becomes 'ned' (Scottish term for a hooligan).
'three' placed around 'at' is 'threate'.
'threate'+'ned'='THREATENED'
(Other definitions for threatened that I've seen before include "was imminent" , "browbeaten" , "Menaced" , "potentially facing extinction" , "Put at risk, endangered" .)