House island boy, first person to be admitted (7)
I believe the answer is:
mansion
'house' is the definition.
(mansion is a kind of house)
'island boy first person to be admitted' is the wordplay.
'island' becomes 'Man' (the Isle of Man is an example).
'boy' becomes 'son' (synonyms).
'first person' becomes 'I' (first-person pronoun).
'to be admitted' is an insertion indicator.
'son' placed around 'i' is 'sion'.
'man'+'sion'='MANSION'
(Other definitions for mansion that I've seen before include "Large building" , "large residence" , "Huge imposing residence" , "Huge abode" , "housing" .)