Warning about Black Prince (6)
I believe the answer is:
albert
'prince' is the definition.
(Prince Albert, husband of Queen Victoria)
'warning about black' is the wordplay.
'warning' becomes 'alert' (alert is a kind of warning).
'about' means one lot of letters goes inside another.
'black' becomes 'b' (abbreviation as in pencil classifications).
'alert' placed around 'b' is 'ALBERT'.
(Other definitions for albert that I've seen before include "Prince with 24" , "Short kind of watch-chain" , "chap" , "Queen Victoria's consort or familiar ex-Taoiseach" , "Queen Victoria's consort or Mr Reynolds, ex-taoiseach" .)