Fierce among fishes, that is! (5)
I believe the answer is:
fiery
'fierce' is the definition.
(thesaurus)
'among fishes that is' is the wordplay.
'among' means one lot of letters goes inside another (inserted letters are placed among others).
'fishes' becomes 'fry' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'fry' enclosing 'ie' is 'FIERY'.
Can you help me to learn more?
(Other definitions for fiery that I've seen before include "Explosive (e.g. temper)" , "Easily angered" , "Choleric" , "Ardent" , "Hot-tempered, passionate" .)