There's very slim chance that she'll need to lose weight (5)
I believe the answer is:
sylph
'weight' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for sylph that I've seen before include "Imaginary (female) being" , "Girl with slender figure" , "Thin, graceful woman" , "Imaginary spirit of the air" , "slender lass!" .)