Lines of ten numbers reversed (6)
I believe the answer is:
sonnet
'lines' is the definition.
(poem of 14 lines)
'ten numbers reversed' is the wordplay.
'numbers' becomes 'nos' (no. is a kind of number).
'reversed' says the letters should be written backwards.
'ten'+'nos'='tennos'
'tennos' backwards is 'SONNET'.
'of' is the link.
(Other definitions for sonnet that I've seen before include "Form of poem" , "Verse form of 14 lines" , "Short poem with 14 lines" , "Fourteen line poem" , "Tonnes (anag.)" .)