Old magistrate on to meet woman in garden (5)
I believe the answer is:
reeve
'old magistrate' is the definition.
(historical type of magistrate or judge)
'on to meet woman in garden' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'to meet' means one lot of letters go next to another.
'woman in garden' becomes 'Eve' (in the Garden of Eden).
're'+'eve'='REEVE'
(Other definitions for reeve that I've seen before include "High official" , "The bailiff to run a rope through" , "Former shire official" , "Saxon magistrate" , "See female bird" .)