Founder of business in Knightsbridge (4)
I believe the answer is:
sink
'founder' is the definition.
(I know that sink is a type of founder)
'of business in knightsbridge' is the wordplay.
'of' indicates the answer is hidden within the clue.
'SINK' can be found hidden inside 'business in knightsbridge'.
(Other definitions for sink that I've seen before include "(Cause to) become submerged" , "Invest capital in - kitchen basin - founder" , "Pocket (billiards)" , "Slump" , "Basin - scupper" .)