Official in royal household diverted abnormal child after intervention by old king (4,11)
I believe the answer is:
lord chamberlain
'official in royal household' is the definition.
'diverted abnormal child after intervention by old king' is the wordplay.
'diverted' indicates anagramming the letters (each letter get diverted elsewhere).
'after intervention by' means one lot of letters goes inside another (to intervene can mean to become involved in).
'old king' becomes 'er' (abbreviation for Edward Rex, various English kings).
'abnormal'+'child'='abnormalchild'
'abnormalchild' anagrammed gives 'lordchamblain'.
'lordchamblain' going around 'er' is 'LORD CHAMBERLAIN'.
(Other definitions for lord chamberlain that I've seen before include "Royal household officer" , "royal household official" , "man in charge at palace" .)