Promises to start painting. Left the borders (7)
I believe the answer is:
pledges
'promises' is the definition.
(pledging is a kind of promising)
'start painting left the borders' is the wordplay.
'start' indicates taking the first letters.
'the borders' becomes 'edges' ('edge' can be a synonym of 'border').
The initial letters of 'painting left' is 'pl'.
'pl'+'edges'='PLEDGES'
'to' is the link.
(Other definitions for pledges that I've seen before include "Vows" , "Promises" , "Loyal toasts" .)