Setter, say, backed up toilet after wiping away runs (8)
I believe the answer is:
gelatine
'backed up toilet after wiping away runs' is the definition.
I can't tell whether this defines the answer.
'setter say' is the wordplay.
'setter' becomes 'gelatin' (substance which sets or hardens).
'say' becomes 'e' (I can't explain this - if you can you should give a lot more credence to this answer).
'gelatin'+'e'='GELATINE'
Can you help me to learn more?
(Other definitions for gelatine that I've seen before include "Setting agent" , "Animal-based gluey substance used in cooking, photography etc" , "It's used in jelly and adhesives" , "Form of glue used in foods" , "Sticky substance obtained from boiled animal tissue" .)