### Clue: Button done on the outside, silk finally threaded through eyelet casually (6,3)
### Answer: DELETE KEY 
 ### Explanation: Definition: Button
A charade of DE ('**D**on**E** on the outside') plus LETEKEY, an envelope ('threaded through') of K ('sil**K** finally') in LETEEY, an anagram ('casually') of 'eyelet'.