Apart from that parent, Mike's left with Ernie? (9)
I believe the answer is:
otherwise
'apart from that' is the definition.
(I've seen this before)
'parent mike's left with ernie?' is the wordplay.
'parent mike's left' becomes 'other' (I can't justify this - if you can you should believe this answer much more).
'with' says to put letters next to each other.
'ernie?' becomes 'Wise' (comedian Ernie Wise).
'other'+'wise'='OTHERWISE'
Can you help me to learn more?
(Other definitions for otherwise that I've seen before include "As an alternative - like white rose" , "White rose (anag.)" , "In other respects or ways" , "Under different conditions" , "Apart from that" .)