Cleansing agent (9)
There are a few possibilities for this:
detergent
(Other definitions for soda that I've seen before include "In the US, a sweet fizzy drink" , "kind of water" , "Drink containing carbonated water and sometimes flavouring" , "Variety of pop" , "for use with whisky?" .)
'cleansing agent' is the definition.
(soap is a kind of cleansing agent)
This is all the clue.
(Other definitions for soap that I've seen before include "A cleansing sort of TV serial" , "bar in Bath?" , "Cleansing TV serial drama" , "Carbolic, perhaps" , "made in a bar?" .)
'cleansing agent' is the definition.
(detergent is a kind of cleansing agent)
This is the entire clue.
(Other definitions for detergent that I've seen before include "Cleaning product" , "Soluble cleansing agent" , "cleaner" , "Get tender (anag)" , "Cleansing substance used for washing clothes" .)