Clue: Opposing sides hosting Arsenal initially pass very slowly (5)
Answer: LARGO
Explanation: Definition: very slowly
 L,R(a., respectively, "left" and "right"/opposing sides) **containing**(hosting) **1st letter of**(… initially) "Arsenal" + GO(to pass/to elapse, as in "As Time Goes By")
Defn: …, as a direction to a musician regarding the played music's tempo.