Eastern chieftains measure US tax office (5)
I believe the answer is:
emirs
'eastern chieftains measure us' is the definition.
I can't judge whether this defines the answer.
'tax office' is the wordplay.
'tax' becomes 'em' (I can't explain this - if you can you should believe this answer much more).
'office' becomes 'IRS' (IRS is an example).
'em'+'irs'='EMIRS'
Can you help me to learn more?
(Other definitions for emirs that I've seen before include "Ruler or chieftain in Arabia)" , "Mires (anag.)" .)