Is flanked a noun

No, the Word 'flanked' is the past participle, past tense of the verb to flank. The past participle of the verb also functions as an adjective.

Examples:

The fireplace was flanked by two overstuffed chairs. (verb)

The flotilla flanked ship came in to port to the cheers of the crowd. (adjective)

The Word 'flank' is both a noun (flank, flanks) and a verb (flank, flanks, flanking, flanked).

The noun form of the verb to flank is the gerund, flanking.