Rubbish salesman a dog perhaps witnessed on the retreat (10)
I believe the answer is:
wastepaper
'rubbish' is the definition.
(I've seen this before)
'salesman a dog perhaps witnessed on the retreat' is the wordplay.
'salesman' becomes 'rep' (sales representative).
'dog perhaps' becomes 'pet' (dogs are often kept as pets).
'witnessed' becomes 'saw' ('see' can be a synonym of 'witness').
'on' is a charade indicator (letters next to each other).
'the retreat' says the letters should be written backwards.
'rep'+'a'+'pet'+'saw'='repapetsaw'
'repapetsaw' reversed gives 'WASTEPAPER'.
(Other definitions for wastepaper that I've seen before include "rubbish" , "Office or domestic bin" .)