Yours truly in Seine swimming around with those from the other side (7)
I believe the answer is:
enemies
'with those from the other side' is the definition.
I can't judge whether this defines the answer.
'yours truly in seine swimming' is the wordplay.
'yours truly' becomes 'em' (I can't explain this - if you can you should believe this answer much more).
'in' means one lot of letters goes inside another.
'swimming' indicates an anagram.
'seine' anagrammed gives 'enies'.
'em' put into 'enies' is 'ENEMIES'.
'around' is the link.
Can you help me to learn more?
(Other definitions for enemies that I've seen before include "Foes, adversaries" , "Foes, opponents" , "Those we fight" , "Foes, hostile groups" .)