One with coat saves energy in freeze (3,4)
I believe the answer is:
ice over
'freeze' is the definition.
(I've seen this in another clue)
'one with coat saves energy' is the wordplay.
'one' becomes 'i' (Roman numeral).
'with' means one lot of letters go next to another.
'coat' becomes 'cover' (I am not sure about this - if you are sure you should believe this answer much more).
'saves' indicates putting letters inside.
'energy' becomes 'e' (physics symbol as in E=mco).
'cover' placed around 'e' is 'ceover'.
'i'+'ceover'='ICE OVER'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for ice over that I've seen before include "Freeze" , "Get very cold outside" .)