Froth starts to spread under dishwasher suddenly (4)
'froth' is the definition.
'starts to spread under dishwasher suddenly' is the wordplay.
'starts to' suggests taking the first letters.
The first letters of 'spread under dishwasher suddenly' is 'SUDS'.
(Other definitions for suds that I've seen before include "Lather", "Soapy froth", "Froth made from soap", "Soapy water", "Foam from detergent".)