Capacity for doing work ends when older and saggy (6)
I believe the answer is:
energy
'capacity' is the definition.
The definition and answer can be both human attributes as well as being singular nouns.
Maybe there's a link between them I don't understand?
'doing work ends when older and saggy' is the wordplay.
I cannot really understand how this works, but
'work' could be 'erg' (an erg is a unit of work done) and 'erg' is located in the answer.
'and' could be 'n' (common abbreviation for 'and') and 'n' is present in the answer.
This explanation may well be incorrect...
'for' is the link.
Can you help me to learn more?
(Other definitions for energy that I've seen before include "Vigour, power" , "Go, zip" , "life" , "Power or vigorous activity" , "Zeal" .)