Improved and finished after the compiler returned! (7)
I believe the answer is:
emended
'improved' is the definition.
(emending is a kind of improving)
'finished after the compiler returned' is the wordplay.
'finished' becomes 'ended' ('ended' can be similar in meaning to 'finished'**).
'after' means one lot of letters go next to another.
'the compiler' becomes 'me' (the person who is writing these clues).
'returned' says the letters should be written in reverse.
'me' written backwards gives 'em'.
'ended' after 'em' is 'EMENDED'.
'and' acts as a link.
(Other definitions for emended that I've seen before include "Made corrections" , "Improved" , "Rectified" , "Made changes to" , "Refined" .)