Warning: its usual partner will have egg on face (6)
I believe the answer is:
beacon
'warning' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for beacon that I've seen before include "warning to others" , "Watch fire" , "This may serve as a warning" , "Navigational guide" , "Warning, guiding, fire" .)