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' indicates putting letters inside (I've seen this in other clues).
'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' ('i.e.' can be a synonym of 'that is').
'fry' going around '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" .)