In a war of attrition, reserves of energy (3)
I believe the answer is:
fat
'reserves of energy' is the definition.
The definition and answer can be both human attributes as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'in a war of attrition' is the wordplay.
'in' indicates a hidden word.
'FAT' can be found hidden inside '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" .)