Child pally with old Mr Banks (5)
I believe the answer is:
minor
'child' is the definition.
(I know that child can be written as minor)
'pally with old mr banks' is the wordplay.
'pally' becomes 'in' (I can't explain this - if you can you should believe this answer much more).
'with' means one lot of letters go next to another.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'banks' is an insertion indicator (bank can mean to surround or border).
'in'+'o'='ino'
'ino' inserted within 'mr' is 'MINOR'.
Can you help me to learn more?
(Other definitions for minor that I've seen before include "Person not yet of legal age" , "Under-age or of little importance" , "Small-scale - child" , "Morris car; tending to melancholy (mus.)" , "Little-known" .)