'I can' belief extraordinarily advantageous (10)
I believe the answer is:
beneficial
'advantageous' is the definition.
(similar in meaning)
'i can belief extraordinarily' is the wordplay.
'extraordinarily' indicates anagramming the letters.
'i'+'can'+'belief'='icanbelief'
'icanbelief' with letters rearranged gives 'BENEFICIAL'.
(Other definitions for beneficial that I've seen before include "profitable" , "Favourable, advantageous" , "helpful" .)