On twenty-four hours earlier, old magistrate (5)
I believe the answer is:
reeve
'old magistrate' is the definition.
(historical type of magistrate or judge)
'on twenty-four hours' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'twenty-four hours' becomes 'eve' (eve is a kind of twenty-four hours).
're'+'eve'='REEVE'
'earlier' is the link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for reeve that I've seen before include "Female flyer" , "Someone with a tale" , "Bird - female of the ruff" , "Pass a rope through a block" , "Historical magistrate" .)