To clean, in effect, every eight years (9)
I believe the answer is:
octennial
'eight years' is the definition.
(I've seen this before)
'to clean in effect' is the wordplay.
'effect' indicates an anagram (I've seen 'effects' mean this).
'to'+'clean'+'in'='tocleanin'
'tocleanin' with letters rearranged gives 'OCTENNIAL'.
'every' is the link.
This may not be right. It may be part of another bit of the clue.
Can you help me to learn more?
(Another definition for octennial that I've seen is " Recurring every eight years".)