Boy secures a large reception room (5)
I believe the answer is:
salon
'reception room' is the definition.
(I've seen this before)
'boy secures a large' is the wordplay.
'boy' becomes 'son' (synonyms).
'secures' is an insertion indicator.
'large' becomes 'l'.
'a'+'l'='al'
'son' enclosing 'al' is 'SALON'.
(Other definitions for salon that I've seen before include "Gallery for works of art -- loans (anag)" , "(Room for) gathering of distinguished people" , "Literary gathering" , "Loans out as a reception room" , "social event" .)