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' means one lot of letters goes inside another (some letters cut their way into a word).
'endlessly' means to remove the last letter.
'regressive' says the letters should be written backwards (to regress is to go backwards).
'taxes' with its final letter removed is 'taxe'.
'c' going inside 'taxe' is 'tcaxe'.
'tcaxe' reversed gives '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" .)