Not one prearranged fight has European later showing swelling (6)
I believe the answer is:
nodule
'swelling' is the definition.
(thesaurus)
'not one prearranged fight has european later' is the wordplay.
'not one' becomes 'no' (eg 'no houses' means 'not one house').
'prearranged fight has' becomes 'dul' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'european' becomes 'e' (abbreviation e.g. EU).
'later' is a charade indicator (letters next to each other).
'no'+'dul'+'e'='NODULE'
'showing' acts as a link.
Can you help me to learn more?
(Other definitions for nodule that I've seen before include "Small protuberance" , "Small round lump" , "Small knot" , "Small wartlike lump" , "Little round lump" .)