Former magistrate's expression of doubt over first-time offender (5)
I believe the answer is:
reeve
'former magistrate's' is the definition.
(I've seen this before)
'expression of doubt over first-time offender' is the wordplay.
I cannot really see how this works, but
'over' could be 're' and 're' is present in the answer.
The remaining letters 'eve' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for reeve that I've seen before include "Scottish bailiff" , "One of Chaucer's Canterbury pilgrims" , "old-fashioned official" , "Steward who told a Tale" , "thread his way through" .)