Supply argument in favour of dream (9)
I believe the answer is:
provision
'supply' is the definition.
(provisioning is a kind of supplying)
'argument in favour of dream' is the wordplay.
'argument in favour of' becomes 'pro' (as in pros and cons).
'dream' becomes 'vision' (I've seen this before).
'pro'+'vision'='PROVISION'
(Other definitions for provision that I've seen before include "Something supplied" , "Catering" , "prior measures made" , "facilities" , "Keep supplied" .)