Taxing single Republican nothing? America! (7)
I believe the answer is:
onerous
'taxing' is the definition.
(I know that taxing can be written as onerous)
'single republican nothing? america' is the wordplay.
'single' becomes 'one' (a single thing - only one).
'republican' becomes 'R' (abbreviation in US politics).
'nothing?' becomes 'o' (looks like zero - 0).
'america' becomes 'US' (United States).
'one'+'r'+'o'+'us'='ONEROUS'
(Other definitions for onerous that I've seen before include "Involving a great deal of effort" , "quite a burden if so" , "Heavy, difficult to do or bear" , "Toilsome" , "Involving much work" .)