Engaged in work regarding tax (2,4)
I believe the answer is:
on duty
'engaged' is the definition.
(thesaurus)
'work regarding tax' is the wordplay.
I cannot quite see how this works, but
'work' could be 'duty' (duty is a kind of work) and 'duty' is found in the answer.
'regarding' could be 'on' (on or regarding a particular topic) and 'on' is located in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'in' acts as a link.
Can you help me to learn more?
(Other definitions for on duty that I've seen before include "(Of soldier) working" , "Engaged in one's regular work" , "What working nurses are" , "As working policemen are" , "Working at a particular time" .)