Nothing but freezer for magistrate (7)
I believe the answer is:
justice
'for magistrate' is the definition.
(synonyms)
'nothing but freezer' is the wordplay.
'nothing but' becomes 'just' (both can mean simply or merely).
'freezer' becomes 'ice' (something which 'freezes').
'just'+'ice'='JUSTICE'
(Other definitions for justice that I've seen before include "Fair play" , "It's dispensed over the bar" , "Rightness" , "right outcome" , "Equity" .)