A Lodge is regular when it adheres to the
Ancient Landmarks of the Order. Adherence is the only measure of regularity. Regularity is not in the gift of any Grand Lodge.
Clandestine means hidden. In the Masonic context a Grand Lodge cannot, by definition, be clandestine if it does not hide its existence.
However a Lodge may be clandestine.
The best way to describe a clandestine Lodge is by quoting the example of the Italian P2 Lodge:
This Lodge met in secrecy, away from the knowledge of the mass membership of its own Masonic jurisdiction, in order to carry out unlawful activities in contravention of the constitutions of its own Grand Lodge.
That is a clandestine Lodge. It is quite specific and the word should not be used loosely or unwisely.
Unfortunately it is often used malevolently. Of course, when that happens, it serves to brand the messenger as either an ignoramus, or a charlatan. Either way he is unquestionably a fool.