Skill I observed in chap describing another planet (7)
I believe the answer is:
martian
'describing another planet' is the definition.
(from Mars)
'skill i observed in chap' is the wordplay.
'skill' becomes 'art' (both can mean a talent).
'observed in' is an insertion indicator.
'chap' becomes 'man' (chap can mean a man).
'art'+'i'='arti'
'arti' inserted within 'man' is 'MARTIAN'.
(Other definitions for martian that I've seen before include "Relating to a particular planet" , "Green man from the Red Planet" , "Sci-fi alien" , "Relating to one planet" , "One living on red planet" .)