Can I evolve the Cleffa from Obsidian Flames into Clefairy?
No. Cleffa doesn't say that it can evolve into Clefairy, and Clefairy doesn't say that it evolves from Cleffa.
Source: Obsidian Flames FAQ; TPCi Rules Team (2023-08-10)

Relevant cards

Note: All applicable cards may not be listed below.
SV3_80
SV3_80