Salesman in capital standing for ruler (7)
I believe the answer is:
emperor
'for ruler' is the definition.
(emperor is a kind of ruler)
'salesman in capital standing' is the wordplay.
'salesman' becomes 'rep' (sales rep).
'in' indicates putting letters inside.
'capital' becomes 'rome' (capital of Italy).
'standing' says the letters should be written backwards (in a down clue, letters stand up).
'rep' placed within 'rome' is 'rorepme'.
'rorepme' written backwards gives 'EMPEROR'.
(Other definitions for emperor that I've seen before include "Title of sovereignty" , "Largest type of penguin" , "Ruler of large territory" , "Purple --; -- penguin" , "Large richly-coloured butterfly" .)