Second bed tax (4)
I believe the answer is:
scot
'tax' is the definition.
(I've seen this before)
'second bed' is the wordplay.
'second' becomes 's' (common abbreviation).
'bed' becomes 'cot' (cot is a kind of bed).
's'+'cot'='SCOT'
(Other definitions for scot that I've seen before include "One from northern UK" , "Stevenson was one" , "Highlander, for example" , "Burns for one" , "Eg Aberdonian" .)