Chap captures a little green man (7)
I believe the answer is:
martian
'man' is the definition.
The answer and definition can be both people as well as being singular nouns.
Maybe you can see a link between them that I can't see?
'chap captures a little green' is the wordplay.
'chap' becomes 'man' (chap can mean a man).
'captures' indicates putting letters inside.
'a little green' becomes 'arti' (I can't justify this - if you can you should give a lot more credence to this answer).
'man' going around 'arti' is 'MARTIAN'.
Can you help me to learn more?
(Other definitions for martian that I've seen before include "Extraterrestrial" , "seeing little green man?" , "Sci-fi alien" , "Relating to one planet" , "One living on red planet" .)