Lunar features in large part of planet, I start to admit (5)
I believe the answer is:
maria
'lunar features in' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'large part of planet i start to admit' is the wordplay.
'large part of planet' becomes 'mar' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'start to' suggests taking the first letters.
The initial letter of 'admit' is 'a'.
'mar'+'i'+'a'='MARIA'
Can you help me to learn more?
(Other definitions for maria that I've seen before include "Girl's name; darker moon areas" , "Heroine of West Side Story" , "Lady" , "More than one plain" , "musical" .)