Clue: Flighty one, tale-teller describing law-breaker's head that is initially displayed on gallows (15)
Answer: FLIBBERTIGIBBET
Explanation: Definition: Flighty one
A charade of FLIBBBER, an envelope ('describing') of L ('**L**aw-breaker's head') in FIBBER ('tale-teller') plus TI ('**T**hat **I**s initially') plus ('displayed on') GIBBET ('gallows').