Immense distance once covered by rodents from the East (7)
I believe the answer is:
stellar
'immense' is the definition.
'distance once covered by rodents from the east' is the wordplay.
'distance once' becomes 'ell' (I've seen this before. I am not sure about the 'once' bit.).
'covered by' indicates putting letters inside.
'rodents' becomes 'rats' (rat is a kind of rodent).
'from the east' shows that the letters should be reversed in order.
'rats' written backwards gives 'star'.
'ell' put within 'star' is 'STELLAR'.
Can you help me to learn more?
(Other definitions for stellar that I've seen before include "boss!" , "characteristic of The Sun, etc" , "Excellent - shining bright?" , "Wonderful" , "of celebrities" .)