Supporting vehicle turning up ahead of ruler (7)
I believe the answer is:
backing
'supporting' is the definition.
(backing is a kind of supporting)
'vehicle turning up ahead of ruler' is the wordplay.
'vehicle' becomes 'cab' (cab is a kind of vehicle).
'turning up' shows that the letters should be reversed in order.
'ahead of ruler' becomes 'king' (I can't justify this - if you can you should believe this answer much more).
'cab' written backwards gives 'bac'.
'bac'+'king'='BACKING'
Can you help me to learn more?
(Other definitions for backing that I've seen before include "change of wind" , "Withdrawing" , "Supporting or betting on" , "funds?" , "Endorsement" .)