Instrument starts to magnify blotches into large dark spots on the moon (7)
I believe the answer is:
marimba
'large dark spots on the moon' is the definition.
I can't judge whether this definition defines the answer.
'instrument starts to magnify blotches into' is the wordplay.
'instrument' becomes 'maria' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'starts to' says to take the initial letters.
'into' means one lot of letters goes inside another.
The first letters of 'magnify blotches' is 'mb'.
'maria' placed around 'mb' is 'MARIMBA'.
Can you help me to learn more?
(Other definitions for marimba that I've seen before include "African xylophone" , "Jazz instrument" , "Xylophone-like instrument from Africa" , "Xylophone-like instr." , "Latin American instrument like a xylophone" .)