Fruitful product of internecine war is losing weight for a short time (9)
I believe the answer is:
nectarine
'weight for a short time' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'fruitful product of internecine war is losing' is the wordplay.
I cannot quite understand how this works, but
'product' could be 'tar' (I have seen 'Product of coal ' mean 'tar' so perhaps 'product' could also mean 'tar') and 'tar' is located in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for nectarine that I've seen before include "Juicy fruit" , "fruity" , "Fruit - aren't nice (anag)" , "Smooth-skinned fruit" , "Luscious fruit" .)