Shock from small, tailless fish (4)
I believe the answer is:
stun
'shock' is the definition.
(both can mean to surprise)
'small tailless fish' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'tailless' means to remove the last letter (the tail of the word is removed).
'fish' becomes 'tuna' (tuna is a type of fish).
'tuna' with its last letter taken away is 'tun'.
's'+'tun'='STUN'
'from' acts as a link.
(Other definitions for stun that I've seen before include "Daze, render semi-conscious" , "Confuse" , "Knock unconscious" , "Concuss; amaze" , "'Shock, daze (4)'" .)