Artist quietly fills one with optimism (6)
I believe the answer is:
hopper
'artist' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'quietly fills one with optimism' is the wordplay.
'quietly' becomes 'pp' (musical abbreviation for pianissimo).
'fills' indicates putting letters inside.
'one with optimism' becomes 'hoer' (I am not sure about this - if you are sure you should believe this answer much more).
'pp' inserted into 'hoer' is 'HOPPER'.
Can you help me to learn more?
(Other definitions for hopper that I've seen before include "Young locust" , "Jumper on one foot" , "Carrier on farm" , "Funnel (eg, feeding grain to mill)" , "US painter" .)