American national swimmer (3)
I believe the answer is:
auk
'swimmer' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'american national' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'national' becomes 'uk' (I can't explain this - if you can you should believe this answer much more).
'a'+'uk'='AUK'
Can you help me to learn more?
(Other definitions for auk that I've seen before include "Great ---, extinct seabird" , "Northern seabird with small narrow wings" , "Stocky diving bird" , "Northern sea-bird with short narrow wings" , "Black-and-white seabird" .)