One sent down for good over unsuccessful coup, originally (7)
I believe the answer is:
lucifer
'one sent down' is the definition.
I can't tell whether this definition defines the answer.
'good over unsuccessful coup originally' is the wordplay.
'good' becomes 'lifer' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'over' indicates putting letters inside.
'originally' suggests taking the first letters.
The first letters of 'unsuccessful coup' is 'uc'.
'lifer' placed around 'uc' is 'LUCIFER'.
'for' is the link.
Can you help me to learn more?
(Other definitions for lucifer that I've seen before include "Light" , "Rebellious angel; a match" , "from the abyss?" , "Devilish sort of match" , "once a match for > Dickens" .)