Square king admitted to fatal error with fish (9)
I believe the answer is:
trafalgar
'square' is the definition.
(I've seen this before)
'king admitted to fatal error with fish' is the wordplay.
'king' becomes 'R' (abbreviation for rex, king in Latin).
'admitted to' indicates putting letters inside.
'error' indicates anagramming the letters.
'with' says to put letters next to each other.
'fish' becomes 'gar' (type of fish).
'fatal' with letters rearranged gives 'tafal'.
'r' put inside 'tafal' is 'trafal'.
'trafal'+'gar'='TRAFALGAR'
(Other definitions for trafalgar that I've seen before include "19th century battle" , "Nelson's eighteen hundred and five victory" , "Battle won by Nelson in 1805" , "London Square and Nelson sea-battle" , "Nelson's last battle" .)