To begin with fish is quite unexpected (9)
I believe the answer is:
startling
'quite unexpected' is the definition.
'startling' can be an answer for 'unexpected' (I've seen this before). I am unsure of the 'quite' bit.
'to begin with fish' is the wordplay.
'to begin' becomes 'start' (synonyms).
'with' says to put letters next to each other.
'fish' becomes 'ling' (ling is a kind of fish).
'start'+'ling'='STARTLING'
'is' is the link.
Can you help me to learn more?
(Other definitions for startling that I've seen before include "Causing shock" , "Unexpected" , "Remarkable" , "Alarming due to rattlings" , "Rattlings of alarming kind" .)