Block of rubbish under potassium and iodine (6)
I believe the answer is:
kibosh
'block' is the definition.
(I know that block can be written as kibosh)
'rubbish under potassium and iodine' is the wordplay.
'rubbish' becomes 'bosh' (I've seen this in another clue).
'under' means one lot of letters go next to another.
'potassium' becomes 'K' (K is the chemical symbol for potassium).
'and' says to put letters next to each other.
'iodine' becomes 'I' (chemical symbol).
'k'+'i'='ki'
'bosh' after 'ki' is 'KIBOSH'.
'of' acts as a link.
(Other definitions for kibosh that I've seen before include "Rubbish - nonsense" , "Make impossible to happen" , "Prevent (something) from happening" , "Finish off" , "Nonsense (slang)" .)