When writing to a clan chief it is normal
to refer to them by their estate and not by their surname and the
English "Esquire" is never used.
At the beginning of a letter you would
ie you are using their territorial designation instead of their surname.
However if a member of a clan is writing to his chief he could put:
On the envelope you would put the full title with territorial
Chisholm of Chisholm or The Chisholm
Colonel Donald Cameron of Lochiel TD (etc with other letters)
The MacNeil of Barra Henry Forbes Irvine of Drum
Peter Barclay of that Ilk
Dugald McTavish of McTavish
If using a verbal address you would use the clan or territorial
designation and not the surname.
eg You would refer to Sir Donald Cameron of Lochiel as simply
Lochiel and if introducing him would say "This is Lochiel".