Bang on about cutting taxes, endlessly regressive (5)
I believe the answer is:
exact
'bang on' is the definition.
(bang on means exactly right)
'about cutting taxes endlessly regressive' is the wordplay.
'about' becomes 'c' (abbreviation for circa).
'cutting' is an insertion indicator (some letters cut their way into a word).
'endlessly' means to remove the last letter.
'regressive' says the letters should be written in reverse (to regress is to go backwards).
'taxes' with its final letter taken off is 'taxe'.
'c' put within 'taxe' is 'tcaxe'.
'tcaxe' back-to-front is 'EXACT'.
(Other definitions for exact that I've seen before include "Spot-on" , "Correct; insist on" , "Absolutely accurate or correct" , "Absolutely right" , "Spot on, precise" .)