Customary tax for second dwelling (4)
I believe the answer is:
scot
'customary tax' is the definition.
'scot' can be an answer for 'tax' (I've seen this before). I'm not sure about the 'customary' bit.
'second dwelling' is the wordplay.
'second' becomes 's' (common abbreviation).
'dwelling' becomes 'cot' (I can't justify this - if you can you should believe this answer much more).
's'+'cot'='SCOT'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for scot that I've seen before include "One from northern UK" , "Native of Glasgow, say" , "Northern member of UK" , "Native of Caledonia" , "Northerner; old tax" .)