Feeble sentence, but economical (8)
I believe the answer is:
slimline
'feeble' is the definition.
Both the definition and answer are adjectives.
Maybe you can see an association between them that I can't see?
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for slimline that I've seen before include "containing little or no sugar" , "Low-calorie" , "Low in calories" , "Only slight" , "Low-cal; slender in build" .)