What idiot put up tax? (5)
I believe the answer is:
tithe
'tax?' is the definition.
(historical tax paid to the church)
'what idiot put up' is the wordplay.
'what' becomes 'eh' (sound used to ask 'what?' or 'pardon?').
'idiot' becomes 'tit' (both can mean a stupid person).
'put up' says the letters should be written backwards.
'eh'+'tit'='ehtit'
'ehtit' reversed gives 'TITHE'.
(Other definitions for tithe that I've seen before include "One-tenth of income paid to church" , "Levy of one tenth of something" , "Church payment" , "It used to decimate income" , "Tax paid to church" .)