Observe fewer are perfectly clean (8)
I believe the answer is:
spotless
'perfectly clean' is the definition.
(I've seen this before)
'observe fewer' is the wordplay.
'observe' becomes 'spot' (I've seen this before).
'fewer' becomes 'less' (associated in meaning).
'spot'+'less'='SPOTLESS'
'are' is the link.
(Other definitions for spotless that I've seen before include "Immaculately clean" , "How Lady Macbeth wanted to be" , "totally innocent" , "Spic-and-span" , "Fully clean" .)