Old magistrate always backed imprisoning English (5)
I believe the answer is:
reeve
'old magistrate' is the definition.
(I've seen this before)
'always backed imprisoning english' is the wordplay.
'always' becomes 'ever' ('ever' can be a synonym of 'always').
'backed' shows that the letters should be reversed in order.
'imprisoning' means one lot of letters goes inside another.
'english' becomes 'e' (abbreviation).
'ever' written backwards gives 'reve'.
'reve' placed around 'e' is 'REEVE'.
(Other definitions for reeve that I've seen before include "Pass a rope through a ring" , "mate of the ruff" , "Former magistrate" , "Former shire official" , "Pass the end of a rope through the bailiff" .)