And I cast the boy in the show (7)
I believe the answer is:
aladdin
'the show' is the definition.
(traditional pantomime)
'and i cast the boy in' is the wordplay.
'cast' is an anagram indicator.
'the boy' becomes 'lad' (lad is a kind of boy**).
'in' indicates putting letters inside.
'and'+'i'='andi'
'andi' with letters rearranged gives 'adin'.
'adin' enclosing 'lad' is 'ALADDIN'.
(Other definitions for aladdin that I've seen before include "Common panto title" , "Pantomime character with wonderful lamp" , "Panto lamp rubber" , "And dial up the owner of a magic lamp" , "Panto lamp owner" .)