Beyond compare, I could be no plainer (9)
I believe the answer is:
nonpareil
'beyond compare i' is the definition.
(someone beyond compare)
'could be no plainer' is the wordplay.
'could be' indicates anagramming the letters.
'no'+'plainer'='noplainer'
'noplainer' anagrammed gives 'NONPAREIL'.
(Other definitions for nonpareil that I've seen before include "A variety of apple" , "Unique type" , "having no match" , "Without equal, unrivalled" , "without peers" .)