Cut down knight in cold Danish city (8)
I believe the answer is:
condense
'cut down' is the definition.
(I've seen this before)
'knight in cold danish city' is the wordplay.
'knight' becomes 'N' (chess abbreviation).
'in' is an insertion indicator.
'cold' becomes 'C' (abbreviation, eg on taps).
'danish city' becomes 'odense' (I've seen this in another clue).
'c'+'odense'='codense'
'n' placed into 'codense' is 'CONDENSE'.
(Other definitions for condense that I've seen before include "Turn from gas to liquid" , "Abbreviate" , "Summarise" , "Make more concentrated, or express in fewer words" , "cut down" .)