Nameless planet with rotation period of another, one of seven (8)
I believe the answer is:
saturday
'nameless planet with rotation' is the definition.
I can't tell whether this definition defines the answer.
'period of another one of seven' is the wordplay.
'period' becomes 'Sat' (Sat is an example).
'of another' becomes 'ur' (I can't explain this - if you can you should give a lot more credence to this answer).
'one of seven' becomes 'day' (I've seen this in another clue).
'sat'+'ur'+'day'='SATURDAY'
Can you help me to learn more?
(Other definitions for saturday that I've seen before include "Samedi or Sabato or Samstag" , "today?" , "Ian McEwan novel about day of the week" , "Day named for the sixth plant from the sun" , "24 hours" .)