Clue: Gather moderate Democrat will oust Republican (6)
Answer: DEDUCE
Explanation: Definition: Gather
 "reduce"(to moderate/to make smaller in degree or size, away from an extreme) with "D"(abbrev. for a member of the Democratic Party) **replacing**(will oust) "R"(abbrev. for a member of the Republican Party).
Defn: …/to conclude.