On top of St Paul's cathedral there's one copper not quite freezing (6)
I believe the answer is:
cupola
'on top of st paul's cathedral there's one' is the definition.
(dome roof, eg on St Paul's Cathedral)
'copper not quite freezing' is the wordplay.
'copper' becomes 'Cu' (chemical symbol).
'not quite' means to remove the last letter.
'freezing' becomes 'polar' (synonyms).
'polar' with its last letter taken away is 'pola'.
'cu'+'pola'='CUPOLA'
(Other definitions for cupola that I've seen before include "Small dome on a roof" , "Dome - furnace" , "fireplace" , "Rounded dome" , "Round domed roof" .)