Perhaps sentence by a magistrate holds up (5)
I believe the answer is:
maybe
'perhaps' is the definition.
('maybe' can be a synonym of 'perhaps')
'sentence by a magistrate holds up' is the wordplay.
'holds' indicates the answer is hidden within the clue.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'ebyam' can be found hidden inside 'sentence by a magistrate'.
'ebyam' reversed gives 'MAYBE'.
(Other definitions for maybe that I've seen before include "Perhaps, possibly" , "Perhaps, might happen" .)