Shock given by singular tailless fish (4)
I believe the answer is:
stun
'shock' is the definition.
(synonyms)
'given by singular tailless fish' is the wordplay.
'given by' means one lot of letters go next to another.
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
'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 final letter taken off is 'tun'.
's' next to 'tun' is 'STUN'.
(Other definitions for stun that I've seen before include "Daze, paralyse" , "'Shock, daze (4)'" , "Daze, scatter senses" , "Anaesthetise" , "Leave speechless" .)