Makes enquiries mostly about tax relating to personal affairs (7)
I believe the answer is:
private
'relating to personal affairs' is the definition.
I can't tell whether this definition defines the answer.
'makes enquiries mostly about tax' is the wordplay.
'makes enquiries mostly' becomes 'prie' (I can't explain this - if you can you should give a lot more credence to this answer).
'about' indicates putting letters inside.
'tax' becomes 'VAT' (Value-Added Tax).
'prie' going around 'vat' is 'PRIVATE'.
Can you help me to learn more?
(Other definitions for private that I've seen before include "Personal or secret" , "See 16" , "One's own" , "Lowest rank in the army" , "(Of a place) secluded" .)