Polish up new stream
I believe the answer is:
burn
'stream' is the definition.
(a burn is a small stream)
'polish up new' is the wordplay.
'polish' becomes 'rub' (polish meaning rub to make shiny).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'rub' backwards is 'bur'.
'bur'+'n'='BURN'
(Other definitions for burn that I've seen before include "Char" , "runner?" , "be consumed" , "Go up in flames; stream" , "Combust" .)