Account hidden in single location where taxes may be avoided (6)
I believe the answer is:
monaco
'location where taxes may be avoided' is the definition.
(Monaco is often used as a tax haven)
'account hidden in single' is the wordplay.
'account' becomes 'ac'.
'hidden in' means one lot of letters goes inside another.
'single' becomes 'mono' (prefix meaning single).
'ac' inserted inside 'mono' is 'MONACO'.
(Other definitions for monaco that I've seen before include "Principality within France" , "State of the Grimaldis" , "Oman Co. (anag)" , "Home of the Grimaldis" , "Prince Rainier's kingdom" .)