It has been suggested to me that Rev Alexander Clark might be related to Donald Clark.
https://www.ecclegen.com/general-index-general-index/ is the best way to find presbyterian ministers. It points to an article about him from Fasti Ecclesiae Scoticanae that says that Rev Alexander Clark was born in Inverness in December 1797 and educated at King's College, Aberdeen, so the Aberdeen University archives will almost certainly have more information about him, including confirmation of his father's name. Unfortunately he died in 1852, so there won't be a death certificate.
Donald Clark's age is given in the 1851 census as 73, which, if accurate, suggests he was born in 1777/1778, not in 1785. In any case, if he was schoolmaster for 50 years, he must have been born earlier than 1785, because he would have been only 18 in 1803, which is a bit young to be a fully-fledged schoolmaster. It also says that he was born in Petty.
The 1841 gives his age as 60. Adults' ages in 1841 were supposed to be rounded down to the nearst 5 years, so he could have been any age from 60 to 64, and if accurate this would mean he was born between 1776 and 1781.
There is a baptism of Alexander Clark, son of Alexander Clark and Martha Rose, in Inverness in December 1797. They also had a son Daniel baptised in 1799. The names Daniel and Donald are used interchangeably, so this suggests that Donald and Alexander were not brothers. Also, Alexander Clark and Martha Rose were married in 1795, which is well after Donald's likely birth.
It might be worth checking the roll of graduates of the University of Aberdeen just in case Donald also studied there.