Driver's aid, of course, for getting the entire amount on the wagon (8)
I believe the answer is:
teetotal
'on the wagon' is the definition.
(on the wagon can mean abstaining from alcohol)
'driver's aid of course' is the wordplay.
'driver's aid' becomes 'tee' (I've seen this in another clue).
'of course' becomes 'total' (I can't explain this - if you can you should give a lot more credence to this answer).
'tee'+'total'='TEETOTAL'
'for getting the entire amount' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for teetotal that I've seen before include "Abstinent from alcohol" , "Abjuring alcohol" , "drunk? No way" , "Avoiding alcohol" , "Completely abstinent" .)