Begin with fish, which is surprising (9)
I believe the answer is:
startling
'is surprising' is the definition.
('startling' can be similar in meaning to 'surprising')
'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'
'which' acts as a link.
(Other definitions for startling that I've seen before include "staggering" , "Alarming due to rattlings" , "Surprising or alarming" , "Jaw-dropping" , "Astonishing or shocking" .)