... who blocks the toilet (6)
I believe the answer is:
throne
'toilet' is the definition.
(I know that toilet can be written as throne)
'who blocks the' is the wordplay.
'who' becomes 'ron' (I can't justify this - if you can you should give a lot more credence to this answer).
'blocks' means one lot of letters goes inside another (the inside letters form a 'blockage').
'ron' inserted into 'the' is 'THRONE'.
Can you help me to learn more?
(Other definitions for throne that I've seen before include "Royal chair" , "heads" , "Ceremonial chair for bishop or king" , "Royalty sits on it" , "Regal seat" .)