Could homoplasy occur within a clade?
According to Furtado and Pessoa (2014):

Homology: A similarity due to common ancestry.

Homoplasy: A similarity not due to common ancestry.

And, according to Futuyma (2006):

Homology: Possession by two or more species of a character state derived, with or without modification, from their common ancestor.

Homoplasy: Possession by two or more species of similar or identical character state that has not been derived by both species from their common ancestor

Therefore, the answer is quite clear: the trait here is an homoplasy, since it's not present in the common ancestor of A and B.

PS: Homoplasy and analogy are not, technically speaking, synonyms. Analogy is a special case of homoplasy.

PPS: A clade is just a monophyletic group. For instance, the hominids form a clade, Mammalia is also a clade, Vertebrata is a clade, even Animalia is a clade. Therefore, a clade can be very exclusive or very inclusive. Because of that, I'm skipping the title of the question ("Could homoplasy occur within a clade?"), whose answer is obviously yes1, and focusing only on your second question: "Is this [image] a case of homoplasy?"

PPPS: Furtado is me, and that quote was translated from the original.

Sources: 


Furtado, G. and Pessoa, F. (2014). Liçoes sobre sete conceitos fundamentais da biologia evolutiva. Brasília: Editora UnB. (translation: On seven fundamental concepts of evolutionary biology)
Futuyma, D. (2006). Evolutionary biology. New York: W.H. Freeman.




1For instance, my metamery and the metamery of an earthworm is an example of homoplasy. Both myself and the earthworm belong to a clade called Animalia. Therefore, there is homoplasy within a clade