Someone swimming around lake east of Luxembourg could be a swimmer (5,4)
I believe the answer is:
lemon sole
'a swimmer' is the definition.
(lemon sole is a kind of fish)
'someone swimming around lake east of' is the wordplay.
'someone swimming' becomes 'lemons' (I can't explain this - if you can you should believe this answer much more).
'around' is an anagram indicator.
'lake' becomes 'l'.
'east' becomes 'e' ('E' can be a synonym of 'east').
'of' becomes 'o'.
'l'+'e'+'o'='leo'
'leo' is an anagram of 'ole'.
'lemons'+'ole'='LEMON SOLE'
'luxembourg could be' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for lemon sole that I've seen before include "Another tasty flatfish like lone citrus fruit" , "Flatfish, valued as food" , "swimmer" , "fish to eat" , "Lose melon, find kind of plaice" .)