Sign seen at hen party? (4)
I believe the answer is:
omen
'sign' is the definition.
(I know that omen is a type of sign)
I don't understand how the rest of the clue works.
(Other definitions for omen that I've seen before include "Foreshadow" , "Forewarning, harbinger" , "Portent, prophetic indicator" , "Foreboding" , "Warning - sign" .)