Threatening sound of cattle crossing English river (8)
I believe the answer is:
lowering
'threatening' is the definition.
(I know that threatening can be written as lowering)
'sound of cattle crossing english river' is the wordplay.
'sound of cattle' becomes 'lowing' (low can mean to moo).
'crossing' means one lot of letters goes inside another.
'english' becomes 'e' (abbreviation).
'river' becomes 'r' (abbreviation for river).
'e'+'r'='er'
'lowing' placed around 'er' is 'LOWERING'.
(Other definitions for lowering that I've seen before include "Reduction - long wire (anag)" , "Making less high" .)