It's back, an article on tax! (5)
I believe the answer is:
tithe
'tax' is the definition.
(thesaurus)
'it's back an article on' is the wordplay.
'it's' becomes 'it' (I've seen this in other clues**).
'back' says the letters should be written backwards.
'an article' becomes 'the' (the 'definite article' part of speech).
'on' means one lot of letters go next to another.
'it' backwards is 'ti'.
'ti'+'the'='TITHE'
(Other definitions for tithe that I've seen before include "Church payment" , "10 per cent levy" , "Impost" , "Tenth part (as tribute)" , "Charge" .)