Learner with an unhappy experience, one who’s invested in his fields (9)
I believe the answer is:
landowner
'one who's invested in his fields' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'learner with an unhappy experience' is the wordplay.
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'with' is a charade indicator (letters next to each other).
'unhappy experience' becomes 'downer' (a downer is a depressing experience).
'l'+'an'+'downer'='LANDOWNER'
Can you help me to learn more?
(Other definitions for landowner that I've seen before include "Man of means" , "Laird" , "One possessing estate" , "One with estates" , "Squire, perhaps" .)