Humiliated PM's predecessor kept in outhouse (6)
I believe the answer is:
shamed
'humiliated' is the definition.
(shame can mean to humiliate or make ashamed)
'pm's predecessor kept in outhouse' is the wordplay.
'pm's predecessor' becomes 'am' (I can't justify this - if you can you should give a lot more credence to this answer).
'kept in' is an insertion indicator.
'outhouse' becomes 'shed' (a shed is an outside building).
'am' put inside 'shed' is 'SHAMED'.
(Other definitions for shamed that I've seen before include "brought into disrepute" , "Embarrassed" , "Dishonoured, disgraced" , "Abashed" , "in disgrace?" .)