Warning enthralling British: German close to the throne (6)
I believe the answer is:
albert
'german close to the throne' is the definition.
(Prince Albert, husband of Queen Victoria)
'warning enthralling british' is the wordplay.
'warning' becomes 'alert' (alert is a kind of warning).
'enthralling' is an insertion indicator.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'alert' going around 'b' is 'ALBERT'.
(Other definitions for albert that I've seen before include "Kind of watch chain" , "Prince with 24" , "Queen Victoria's consort or familiar ex-Taoiseach" , "German prince" , "and 7: French missionary" .)