Mortice plugged by centre of key turns fast (8)

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

An anagram (‘turns’) of ‘mortice’ plus E (‘centre of k E y’). ‘Plugged by’ suggests an envelope, which it is, but with ‘turns’, placed where it is, ‘plugged by’ is unnecessary for the wordplay.