Adviser to Charlemagne Gauls caution regularly (6)

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

{g}A{u}L{s} C{a}U{t}I{o}N [regularly]. This was Alcuin of York (735-804) scholar, clergyman, poet, and teacher. I didn’t know about his association with Charlemagne but I was aware of him from the age of about 7 when I attended a prep school named in his honour.