No, it is because 'clach' is feminine and you use "a'" with words beginning with b, c, g, p, m (see the notes for Feminine Articles). At the risk of a non-PC mneomic, I remember this as "Beautiful Cute Girls Put on Make-up" ('Girls' reminds me it is for Femninine articles) and I changed the mnemonic provided in the notes for masculine articles (Big Fat Members of Parliament) to "Big Fat Male Politicians" (so "Male" reminds me it is for masculine articles). No offence is intended with either mnenomic! The masculine article for words begining with b, f, m, p is "am" and pretty much every thing else is "an" or "an t-" (the latter for words beginnning with a vowel). All that said, the notes are your friend and take precedence over anything I've said here: https://www.duome.eu/tips/en/gd.