Boast new top (5)
I believe the answer is:
crown
'top' is the definition.
(crown is a kind of top)
'boast new' is the wordplay.
'boast' becomes 'crow' (crow is a kind of boast).
'new' becomes 'n'.
'crow'+'n'='CROWN'
(Other definitions for crown that I've seen before include "Headdress, coin" , "Hit on the head (informal)" , "'Uneasy lies the head that wears a . . . . .'" , "Symbol of royalty" , "Symbol of monarchy" .)