Wilder sibling's creation? (7)
I believe the answer is:
genesis
'creation?' is the definition.
(thesaurus)
'wilder sibling's' is the wordplay.
'wilder' becomes 'gene' (gene wilder).
'sibling' becomes 'sis' (I have seen 'Little sibling ' mean 'sis' so perhaps 'sibling' could also mean 'sis').
'gene'+'sis'='GENESIS'
(Other definitions for genesis that I've seen before include "Origin; Bible book" , "Beginning (in the Bible)" , "Origin or formation of something" , "First book of the Bible" , "Beginning the holy book" .)