Sign displayed by topless females (4)
I believe the answer is:
omen
'sign' is the definition.
(I know that omen is a type of sign)
'topless females' is the wordplay.
'topless' means to remove the first letter.
'females' becomes 'women' (woman is a kind of female).
'women' with its first letter taken off is 'OMEN'.
'displayed by' is the link.
(Other definitions for omen that I've seen before include "Portent, prophetic indicator" , "Portent of things to come" , "Straw in the wind" , "Augur" , "Foreshadow" .)