Product of soap operas, you would guess? Only EastEnders (4)
I believe the answer is:
suds
'product of soap' is the definition.
(soap suds)
'operas you would guess? only eastenders' is the wordplay.
'only eastenders' suggests the final letters (letters at the \'eastern\' ends of the words).
The last letters of 'operas you would guess' are 'SUDS'.
(Other definitions for suds that I've seen before include "Soapy water" , "Froth made from soap" , "Froth of soap and water" , "Soapy froth" , "Soap bubbles; scum" .)