... unnecessarily as Countdown is finally going (3)

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

I can’t see the wordplay here.  I think it might relate to the entry, ONSET , at 6 across, but if we take out the letters N and S (l ast letters of [finally] A S or I S and COUNTDOW N ) we are left with OET , which I can’t link to