Wayward prisoner locked up by Mo (10)
I believe the answer is:
inconstant
'wayward' is the definition.
(thesaurus)
'prisoner locked up by mo' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'locked up by' indicates putting letters inside.
'mo' becomes 'instant' (both can mean a short period of time).
'con' put within 'instant' is 'INCONSTANT'.
(Other definitions for inconstant that I've seen before include "Often changing" , "Variable, irregular" , "Fickle" .)