It's seventy feet but only one metre (6)
I believe the answer is:
sonnet
'metre' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for sonnet that I've seen before include "Fourteen line poem" , "14-line verse" , "Shelley's Ozymandias, for example" , "14-line poem" , "Poetic fourteen-liner" .)