Restore salesman's demeanour? (6)
I believe the answer is:
repair
'restore' is the definition.
('repair' can be a synonym of 'restore')
'salesman's demeanour?' is the wordplay.
'salesman' becomes 'rep' (I've seen this before).
'demeanour?' becomes 'air' (air is a kind of demeanour).
'rep'+'air'='REPAIR'
(Other definitions for repair that I've seen before include "Mend, put right" , "darn?" , "Correct" , "Patch up" , "Travel or proceed toward some place" .)