Extremely reliable, the first female magistrate in history (5)
I believe the answer is:
reeve
'magistrate in history' is the definition.
(historical type of magistrate)
'extremely reliable the first female' is the wordplay.
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'the first' is a charade indicator (letters next to each other).
'female' becomes 'eve' (Eve is an example).
'reliable' with its centre taken out is 're'.
're'+'eve'='REEVE'
(Other definitions for reeve that I've seen before include "[PILGRIM in the CANTERBURY TALES]" , "See female bird" , "Feudal official of medieval England" , "old-fashioned official" , "Comic actor" .)