Clue: Metrical form, hybrid, having to conform to its rules outwardly? (6)
Answer: SCAZON
Explanation: Definition: Metrical form
ZO (the famous and cruciverbially useful hybrid cattle) in SCAN (to follow prosodic rules). A Scazon is also known as a choliamb, whose definition will test your knowledge of poetical terminology: "a variety of iambic trimeter having a spondee for an imabus at the sixth foot". The names come from the Greek for "limping" or "lame".