Check fellow is cleaner (9)
I believe the answer is:
detergent
'cleaner' is the definition.
(detergent is a kind of cleaner)
'check fellow' is the wordplay.
'check' becomes 'deter' (I've seen this in other clues).
'fellow' becomes 'gent' (I've seen this before).
'deter'+'gent'='DETERGENT'
'is' acts as a link.
(Other definitions for detergent that I've seen before include "Soluble cleanser" , "Synthetic cleaning substance for washing machines" , "Cleaning product" , "cleaner" , "help with the washing-up" .)