You're taking sides after American's affair (5)
I believe the answer is:
amour
'affair' is the definition.
(synonyms)
'you're taking sides after american's' is the wordplay.
'taking sides' says to take the centre (the letters at the sides are taken off).
'after' says to put letters next to each other.
'american' becomes 'Am' (abbreviation in some contexts).
The middle letters of 'youre' are 'our'.
'our' after 'am' is 'AMOUR'.
(Other definitions for amour that I've seen before include "Fling" , "(Illicit) sexual relationship" , "Love-affair" , "Illicit love" , "Romantic liaison" .)