Impression, not good, full of mostly rubbish ability to produce desired results (8)
I believe the answer is:
efficacy
'impression' is the definition.
The answer and definition can be both human attributes as well as being singular nouns.
Maybe you can see an association between them that I can't see?
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for efficacy that I've seen before include "Ability to produce required result" , "The ability to produce a desired or intended result" , "Being able to work" , "The power to produce an effect" , "Force" .)