Article in More! not taking sides (3)
I believe the answer is:
the
'article' is the definition.
(the definite article in English)
'more not taking sides' is the wordplay.
'more' becomes 'other' (synonyms).
'not taking sides' indicates the central letters (the external or side letters are removed).
The middle of 'other' is 'THE'.
'in' acts as a link.
(Other definitions for the that I've seen before include "and 24 Down: Similar things" , "French drink" , "That particular one" , "Base of ever-expanding nursery rhyme" , "Finis" .)