Planet has room for a boy (8)
I believe the answer is:
marshall
'boy' is the definition.
'planet has room' is the wordplay.
'planet' becomes 'mars' (Mars is a planet).
'has' means one lot of letters go next to another.
'room' becomes 'hall' (hall is a kind of room).
'mars'+'hall'='MARSHALL'
'for a' is the link.
(Other definitions for marshall that I've seen before include "Author of European post-war recovery plan" , "Law officer in the old West" , "Scottish sculptor" , "- - plan (post WW2)" .)