He may have stacks of work to do (7,5)
I believe the answer is:
'do' is the definition.
Both the definition and answer are singular nouns.
Perhaps there's a link between them I don't understand?
'he may have stacks of work' is the wordplay.
I cannot really see how this works, but
'he' could be 'hi' and 'hi' is found in the answer.
'work' could be 'w' (physics abbreviation) and 'w' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
'to' is the link.
Can you help me to learn more?
(Other definitions for chimney sweep that I've seen before include "One to keep the home fires burning?" , "Sooty person" , "Remover of soot" , "Sooty chap?" , "Flue clearer" .)