At work, - but raising no tax (2,4)
I believe the answer is:
on duty
'at work' is the definition.
(I've seen this before)
'raising no tax' is the wordplay.
'raising' is a reversal indicator.
'tax' becomes 'duty' (duty is a kind of tax).
'no' reversed gives 'on'.
'on'+'duty'='ON-DUTY'
'but' is the link.
(Other definitions for on duty that I've seen before include "(Of soldier) working" , "Working at a particular time" , "As working policemen are" , "What working nurses are" , "Engaged in one's regular work" .)