They say the setter used to be hard -- that's bunkum! (7)
I believe the answer is:
eyewash
'that's bunkum' is the definition.
(thesaurus)
'they say the setter used to be hard' is the wordplay.
'they say' indicates a 'sounds like' (homophone) clue.
'the setter' becomes 'I' (the person that set the crossword).
'used to be' becomes 'was' (eg 'he was...' means 'he used to be...').
'hard' becomes 'h' (abbreviation used in pencil classifications).
'i' sounds like 'eye'.
'eye'+'was'+'h'='EYEWASH'
(Other definitions for eyewash that I've seen before include "rot?" , "Slangy nonsense or optical cleanser" , "Nonsense (colloq.)" , "Nonsense (informal)" , "Nonsense; cleansing lotion" .)