Washer might go through this hole in drain (4)
I believe the answer is:
soap
'washer might go through this' is the definition.
(someone washing up might use soap)
'hole in drain' is the wordplay.
'hole' becomes 'O'.
'in' indicates putting letters inside.
'drain' becomes 'sap' (sap can mean to suck or drain).
'o' going within 'sap' is 'SOAP'.
(Other definitions for soap that I've seen before include "TV series of cleansing sort" , "Cleaner - long-running TV show" , "Popular TV programme (which cleans?)" , "Coronation Street?" , "Serialised drama" .)