Group from the south races alien in green (7)
I believe the answer is:
octette
'group' is the definition.
The answer is an example.
'from the south races alien in green' is the wordplay.
'from the south' says the letters should be written in reverse (in a down clue, letters go up).
'races' becomes 'tt' (Isle of Man Tourist Trophy races).
'alien' becomes 'et' (extra terrestrial).
'in' indicates putting letters inside.
'green' becomes 'eco' (abbreviation for ecological).
'tt'+'et'='ttet'
'ttet' placed into 'eco' is 'ettetco'.
'ettetco' in reverse letter order is 'OCTETTE'.
(Other definitions for octette that I've seen before include "Noted work" , "Group of eight" , "Eight musicians" .)