Opposing sides hosting Arsenal initially pass very slowly (5)
I believe the answer is:
largo
'very slowly' is the definition.
(musical direction)
'opposing sides hosting arsenal initially pass' is the wordplay.
'opposing sides' becomes 'LR' (left and right).
'hosting' means one lot of letters goes inside another (inserted letters are welcomed in or hosted).
'initially' says to take the initial letters.
'pass' becomes 'go' (synonyms).
The initial letter of 'arsenal' is 'a'.
'lr' enclosing 'a' is 'lar'.
'lar'+'go'='LARGO'
(Other definitions for largo that I've seen before include "Slow movement in music" , "Deliberately" , "In a slow tempo (mus.)" , "Goldfinger part" , "major key" .)