One's turned long distances into rhetorical devices (7)
I believe the answer is:
similes
'rhetorical devices' is the definition.
(simile is a kind of rhetorical device)
'one's turned long distances' is the wordplay.
'one's' becomes 'I's' (Roman numeral).
'turned' shows that the letters should be reversed in order.
'long distances' becomes 'miles' (I've seen this in another clue. I am not sure about the 'long' bit.).
'is' backwards is 'si'.
'si'+'miles'='SIMILES'
'into' acts as a link.
Can you help me to learn more?
(Other definitions for similes that I've seen before include "Figures of speech such as 13 across and 4 down" , "Figure of speech" , "Figures of speech showing resemblance between things" , "Missile directed at figures of speech" .)