A lot of support in Oz – you can count on it (6)
I believe the answer is:
abacus
'count on it' is the definition.
(I've seen this in another clue)
'a lot of support in oz you' is the wordplay.
'lot of' means to remove the last letter (a lot of the word but not all of it).
'support' becomes 'back' (to back is to support or endorse).
'in oz you' becomes 'us' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'back' with its last letter removed is 'bac'.
'a'+'bac'+'us'='ABACUS'
'can' is the link.
Can you help me to learn more?
(Other definitions for abacus that I've seen before include "summer once?" , "this adds up" , "not something we now count on" , "one counts on it" , "It will support the architrave" .)