For instance, essay about moon composed for this subject? (9)
I believe the answer is:
astronomy
'this subject?' is the definition.
(astronomy is a kind of subject)
'for instance essay about moon composed' is the wordplay.
'for instance' becomes 'as'.
'essay' becomes 'try' ('try' can be a synonym of 'essay').
'about' is an insertion indicator.
'composed' indicates anagramming the letters.
'moon' anagrammed gives 'onom'.
'try' going around 'onom' is 'tronomy'.
'as'+'tronomy'='ASTRONOMY'
'for' is the link.
(Other definitions for astronomy that I've seen before include "Study of stars etc" , "Mayors not involved in study of the stars" , "Tory mason engaged in study of stars" , "Scientific study of the universe" , "Stargazing" .)