Cleaner man put off at the outset (9)
I believe the answer is:
detergent
'cleaner' is the definition.
(chemical that cleans)
'man put off at the outset' is the wordplay.
'man' becomes 'gent' (gent is a kind of man).
'put off' becomes 'deter' (both can mean to discourage).
'at the outset' means one lot of letters go next to another.
'gent' after 'deter' is 'DETERGENT'.
(Other definitions for detergent that I've seen before include "Soluble cleansing agent" , "Synthetic cleaning substance for washing machines" , "Cleansing substance used for washing clothes" , "Soapless cleanser" , "cleaner" .)