One partaking thematically is also a contributor (6)
I believe the answer is:
feeder
This might be a double definition.
'one partaking thematically is' is the first definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'a contributor' is the second definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'also' is the link.
Can you help me to learn more?
(Other definitions for feeder that I've seen before include "Garden dispenser for birds" , "Bird food container" , "Supplier" , "electrical conductor" , "Plant or animal that takes food in certain way" .)