You mean why not "in a/the deficit"? "in deficit" is a set expression. There are quite a number of idiomatic preposition+noun combinations where normally countable nouns drop the article:
to hand in goal in person on deck on site etc. etc.
"adjectival" is normally an adjective. It's true that it does occasionally seem to be used as a noun, so your "an adjectival" may not be impossible after all.