How many letters does the top answer for “supported by both sides” have?

The most common answer is bipartisan (10 letters).

Crossword clue: supported by both sides

1 answer found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

bipartisan 10 letters wn-def

supported by both sides

concave on both sides 50% similar → biconcave

flabellate on both sides. 50% similar → biflabellate