Surprising grade in, so impressive (9)
I believe the answer is:
grandiose
'impressive' is the definition.
(similar in meaning)
'surprising grade in so' is the wordplay.
'surprising' indicates an anagram.
'grade'+'in'+'so'='gradeinso'
'gradeinso' with letters rearranged gives 'GRANDIOSE'.
(Other definitions for grandiose that I've seen before include "affected" , "Ridiculously impressive or imposing" , "Pretentiously imposing -- organised (anag)" , "Bombastic" , "Impressive, magnificent" .)