Small bit of criticism finally follows tax office (4)
I believe the answer is:
atom
'small bit' is the definition.
(I've seen this in another clue)
'criticism finally follows tax office' is the wordplay.
'finally' says to take the final letters.
'follows' means one lot of letters go next to another.
'tax office' becomes 'ato' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The last letter of 'criticism' is 'm'.
'm' after 'ato' is 'ATOM'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for atom that I've seen before include "Particle; molecule" , "tiny thing" , "Small amount of matter" , "Minute fragment" , "The tiniest particle" .)