Begin with fish? Amazing! (9)
I believe the answer is:
startling
'amazing' is the definition.
(thesaurus)
'begin with fish?' is the wordplay.
'begin' becomes 'start' ('start' can be a synonym of 'begin').
'with' is a charade indicator (letters next to each other).
'fish?' becomes 'ling' (ling is a kind of fish).
'start'+'ling'='STARTLING'
(Other definitions for startling that I've seen before include "Unexpected" , "Surprising or alarming" , "Astonishing or shocking" , "Remarkable" , "Producing sudden alarm" .)