Speak out for zero taxation (5)
I believe the answer is:
orate
'speak out' is the definition.
(I've seen this in another clue)
'zero taxation' is the wordplay.
'zero' becomes 'O' (O resembles 0).
'taxation' becomes 'rate' (rate is a kind of taxation).
'o'+'rate'='ORATE'
'for' is the link.
(Other definitions for orate that I've seen before include "Lecture" , "Give speech" , "Speak publicly" , "Deliver a speech" , "Erato (anag.)" .)