He'll give a fellow a bit of work (5)
I believe the answer is:
donor
'he'll' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'give a fellow a bit of work' is the wordplay.
'give' means one lot of letters go next to another.
'a fellow' becomes 'don' (both can mean an academic).
'a bit of work' becomes 'or' (I can't justify this - if you can you should give a lot more credence to this answer).
'don'+'or' is 'DONOR'.
Can you help me to learn more?
(Other definitions for donor that I've seen before include "Organ giver?" , "Philanthropist" , "contributor" , "One giving something" , "A giver, perhaps of blood" .)