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' means one lot of letters goes inside another.
'mo' becomes 'instant' (both can mean a short period of time).
'con' put into 'instant' is 'INCONSTANT'.
(Other definitions for inconstant that I've seen before include "Often changing" , "Variable, irregular" , "Fickle" .)