Order policemen, in extremes of temperature, to retreat (5)
I believe the answer is:
edict
'order' is the definition.
('edict' can be a synonym of 'order')
'policemen in extremes of temperature to retreat' is the wordplay.
'policemen' becomes 'cid' (I've seen this in other clues).
'in' is an insertion indicator.
'extremes of' says to hollow out the word (remove centre letters).
'to retreat' says the letters should be written backwards.
'temperature' with its centre removed is 'te'.
'cid' inserted inside 'te' is 'tcide'.
'tcide' written backwards gives 'EDICT'.
(Other definitions for edict that I've seen before include "Formal order" , "Formal proclamation" , "Authoritative ruling" , "Legally binding command" , "Cited (anag) - proclamation" .)