Thank you all for your suggestions.
I also thought that it might be Clr, meaning "clerk".
There were no other abbreviations in the pages that I scanned.
However, a previous entry in the Burton register refers to the burial, on 18th November 1618, of a clerk to Mr John, Mr George and Sir William Massie.
William first appears in the Burton register in 1622, when one of his daughters was baptized, so he could have been the deceased clerk's successor.
William's 1629 Cheshire will describes him as a "gent", rather than a clerk, so I had assumed that he was not ordained, but rather a clerk in the modern sense. There is also no record that he attended a university or took holy orders in the clergy databases.
It would be intriguing to know how he entered the Masseys' service. Wrenbury his home parish, is some distance from the Wirral. I looked, but could find no family connection, by blood or marriage, that might have brought him too their attention.
regards
Andrew