Terribly mean church is threat (6)
I believe the answer is:
menace
'is threat' is the definition.
(I've seen this before)
'terribly mean church' is the wordplay.
'terribly' indicates anagramming the letters.
'church' becomes 'CE' (abbreviation for Church of England).
'mean' with letters rearranged gives 'mena'.
'mena'+'ce'='MENACE'
(Other definitions for menace that I've seen before include "Intimidate" , "serious nuisance" , "Intimidation" , "A threatening quality" , "peril" .)