Planet Earth enthusiast with something to write about (7)
I believe the answer is:
neptune
'planet' is the definition.
(I know that Neptune is a planet)
'earth enthusiast with something to write about' is the wordplay.
'earth' becomes 'E'.
'enthusiast' becomes 'nut' (nut can mean an enthusiast or fan).
'with' says to put letters next to each other.
'something' becomes 'pen' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'to write about' says the letters should be written backwards (I've seen 'written about' mean this).
'e'+'nut'+'pen'='enutpen'
'enutpen' back-to-front is 'NEPTUNE'.
Can you help me to learn more?
(Other definitions for neptune that I've seen before include "Roman sea god - planet" , "Sea god - outer planet" , "Holst composed it" , "extract from The Planets" , "heavenly body" .)