A good one can always grow better (8)
I believe the answer is:
gardener
'better' is the definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for gardener that I've seen before include "One grows" , "Plant tender" , "Plant and veg grower" , "He tends flowers and shrubs" , "Garnered by worker on cultivated ground" .)