Discourage fellow cleaner (9)
I believe the answer is:
detergent
'cleaner' is the definition.
(detergent is a kind of cleaner)
'discourage fellow' is the wordplay.
'discourage' becomes 'deter' ('deter' can be a synonym of 'discourage').
'fellow' becomes 'gent' (I've seen this before).
'deter'+'gent'='DETERGENT'
(Other definitions for detergent that I've seen before include "Soluble cleanser" , "Synthetic cleaning substance for washing machines" , "Soluble cleansing agent" , "Get tender (anag)" , "Soapless cleanser" .)