Turning point in tax legislation (4)
I believe the answer is:
axle
'turning point' is the definition.
(a rotating part of a machine or vehicle)
'in tax legislation' is the wordplay.
'in' says the answer is hidden in the clue.
'AXLE' is hidden within 'tax legislation'.
(Other definitions for axle that I've seen before include "It's pivotal" , "Pin on which a wheel turns" , "Rod to carry a wheel" , "Rod connecting wheels" , "Wheel rotates around it" .)