He has a lot of interest in his work (6)
I believe the answer is:
usurer
'he' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'a lot of interest in his work' is the wordplay.
'a lot of' means to remove the last letter (most of the word but not all of it).
'interest' becomes 'usury' (usury is a kind of interest).
'in his work' becomes 'er' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'usury' with its last letter taken off is 'usur'.
'usur'+'er'='USURER'
'has' acts as a link.
Can you help me to learn more?
(Other definitions for usurer that I've seen before include "Grasping money lender" , "Shylock, for example" , "Shylock's profession" , "Grasping money-lender" , "Iniquitous moneylender" .)