In a war of attrition, reserves of energy (3)
I believe the answer is:
fat
'reserves of energy' is the definition.
The answer and definition can be both human attributes as well as being singular nouns.
Maybe there's an association between them I don't understand?
'in a war of attrition' is the wordplay.
'in' indicates the answer is hidden within the clue.
'FAT' is hidden within 'war of attrition'.
Can you help me to learn more?
(Other definitions for fat that I've seen before include "Adipose tissue" , "Naturally oily substance" , "Obese, having spare tyre etc" , "Profitable" , "Excess body weight" .)