Watch out (6)
(Other definitions for danger that I've seen before include "Peril, exposure to harm" , "Peril in garde" , "Peril, jeopardy" , "Peril, hazard" , "Red flag, for example" .)
'watch out' is the definition.
(bewaring is a kind of watching out)
This is all the clue.
(Other definitions for beware that I've seen before include "Be alert for dangers" , "Wee bra (anag)" , "Be on one's guard" , "cave" , "Be cautious about" .)