Snarl meant to keep up road tax share (9)
I believe the answer is:
allotment
'share' is the definition.
(allotment is a kind of share)
'snarl meant to keep up road tax' is the wordplay.
'snarl' indicates an anagram (I've seen 'snarled' mean this (to snarl can mean to entangle or twist)).
'to keep' is an insertion indicator.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'road tax' becomes 'toll' (I've seen this in another clue).
'toll' reversed gives 'llot'.
'meant' is an anagram of 'ament'.
'ament' enclosing 'llot' is 'ALLOTMENT'.
(Other definitions for allotment that I've seen before include "share in growing concern?" , "Portion, distribution" , "Where vegetables may be produced" , "patch" , "grant?" .)