“The King of Siam,” cast many to appear in (5)
I believe the answer is:
midas
'the king' is the definition.
(I've seen this before)
'siam cast many to appear in' is the wordplay.
'cast' is an anagram indicator.
'many' becomes 'd' (**).
'to appear in' is an insertion indicator.
'siam' is an anagram of 'mias'.
'mias' going around 'd' is 'MIDAS'.
'of' acts as a link.
(Other definitions for midas that I've seen before include "Legendary king with golden touch" , "King who turned what he touched to gold" , "One with a golden touch" , "Greek mythological king with money-making touch" , "Gold-making king" .)