Sign of females losing weight (4)
I believe the answer is:
omen
'sign' is the definition.
(I know that omen is a type of sign)
'females losing weight' is the wordplay.
'females' becomes 'women' (woman is a kind of female).
'losing' is a deletion indicator.
'weight' becomes 'w' (short for weight).
'women' with 'w' removed is 'OMEN'.
'of' is the link.
(Other definitions for omen that I've seen before include "Harbinger, augury" , "Forecast" , "Sign of something about to happen" , "Warning - sign" , "Prophetic significance" .)