Set for scan? Why? (5)
I believe the answer is:
ready
'set' is the definition.
('ready' can be a synonym of 'set')
'scan? why?' is the wordplay.
'scan?' becomes 'read' (scanning is a kind of reading).
'read'+'y'='READY'
'for' is the link.
(Other definitions for ready that I've seen before include "At hand" , "Prepare money" , "Suitably prepared" , "bread" , "Willing (to do something); cash" .)