Amazing statement supported by expert (10)
I believe the answer is:
remarkable
'amazing' is the definition.
(thesaurus)
'statement supported by expert' is the wordplay.
'statement' becomes 'remark' (remark is a kind of statement**).
'supported by' is a charade indicator (letters next to each other) (letters go below - in down clue).
'expert' becomes 'able' (I've seen this in other clues).
'remark'+'able'='REMARKABLE'
(Other definitions for remarkable that I've seen before include "A wrongly-graded exam paper" , "Extraordinary" , "unusual" , "Striking" , "surprising?" .)