Aloud, runs down old preacher (4)
I believe the answer is:
knox
'old preacher' is the definition.
(16th-century preacher John Knox)
'aloud runs down' is the wordplay.
'aloud' shows a homophone (sound like).
'runs down' becomes 'knocks' (knocks over).
'knocks' is a homophone of 'KNOX'.
(Other definitions for knox that I've seen before include "preacher" , "John --, Scottish religious reformer" , "Scotttish churchman" , "See 24 Down" , "See 21" .)