Washing substance (4)
I believe the answer is:
soap
'washing substance' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for soap that I've seen before include "cake?" , "Cleansing TV serial drama" , ". . . . opera, sentimentalised radio or tv series" , "Sudsy TV show?" , "Carbolic, perhaps" .)