Support the whole number on the wagon (8)
I believe the answer is:
teetotal
'on the wagon' is the definition.
(thesaurus)
'support the whole number' is the wordplay.
'support' becomes 'tee' (tee is a kind of support).
'the whole number' becomes 'total' (I've seen this before**).
'tee'+'total'='TEETOTAL'
(Other definitions for teetotal that I've seen before include "Sober" , "Never drinking alcohol" , "Abstaining completely from alcoholic drinks" , "Abstention from alcoholic drink" , "drunk? No way" .)