All square now with Internal Revenue Service (4)
I believe the answer is:
even
'all square' is the definition.
(I've seen this before)
'with internal revenue service' is the wordplay.
'EVEN' is hidden within the letters.
However, I'm not sure how the hidden word is indicated.
'now' acts as a link.
Can you help me to learn more?
(Other definitions for even that I've seen before include "Deep crisp snow seen on Boxing Day also this" , "Flat, level" , "end of the day, reading poetry" , "Peaceful" , "like the numbers on two sides here" .)