Boasted about knight made king? (7)
I believe the answer is:
crowned
'made king?' is the definition.
'boasted about knight' is the wordplay.
'boasted' becomes 'crowed' (crowing is a kind of boasting).
'about' is an insertion indicator.
'knight' becomes 'n' (chess).
'crowed' placed around 'n' is 'CROWNED'.
(Other definitions for crowned that I've seen before include "Capped" , "wreath on head" , "Formally made monarch" , "Invested as monarch" , "filled with foaming ale" .)