Celebrate about One Direction's first track (6)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

SING (celebrate), around I (one in Roman numerals) + first letter of D [irection]. The boy band One Direction are often abbreviated to 1D, but that isn’t necessary to solve the clue. Siding = a section of railway track that isn’t part of the main route.