Describing a noun as “like an indefinite” (كالنكرة) does not remove it from the grammatical category of definites. The comparison concerns meaning and reference: exact individuation in the external world is unnecessary.
Grammar: «الذِّئْبُ» is a noun made definite by al-.
Interpretation: the fear concerns a possible wolf of that kind.
Common error: reducing this to pure generic essence, or treating it as a claim about every wolf.
The speaker's purpose and the discourse allow us to distinguish one known individual, a nature, and a mentally available potential member of a class.