Save time on fastening (6)
I believe the answer is:
button
'fastening' is the definition.
(I know that button is a type of fastening)
'save time on' is the wordplay.
'save' becomes 'but' ('but' can mean 'save' or 'except').
'time' becomes 't' (abbreviation).
'but'+'t'+'on'='BUTTON'
(Other definitions for button that I've seen before include "Fastener - English F1 racing driver" , "bit of the mattress?" , "Bright as a - -" , "Worthless, they say" , "Fastener used on clothes etc" .)