Showing great spirit, that is, among young swimmers (5)
I believe the answer is:
fiery
'showing great spirit that' is the definition.
The definition and answer are not the same part of speech.
'among young swimmers' is the wordplay.
'among' indicates putting letters inside (I've seen this in other clues).
'young' becomes 'fry' (I've seen this in another clue).
'swimmers' becomes 'ie' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'fry' placed around 'ie' is 'FIERY'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for fiery that I've seen before include "Easily angered" , "Impetuous; hot-tasting" , "Intensely hot" , "Inflamed" , "Quick-tempered" .)