Years working the borders of Kansas. Years and years (5)

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

Y (years), ON (working), K{ansa}S [the borders of…]. This dates from the 1960s but there’s no definitive explanation of how it came into being. The most popular suggestion seems to be that it’s in some way derived from ‘donkey’s years’, an expression that also means ‘a long time’.