Oscar popped in to unveil outlandish theory (9)
I believe the answer is:
evolution
'theory' is the definition.
(I have seen 'Darwinian theory ' mean 'evolution' so perhaps 'theory' could also mean 'evolution')
'oscar popped in to unveil outlandish' is the wordplay.
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'popped in' is an insertion indicator.
'outlandish' indicates anagramming the letters.
'to'+'unveil'='tounveil'
'tounveil' is an anagram of 'evolutin'.
'o' placed within 'evolutin' is 'EVOLUTION'.
(Other definitions for evolution that I've seen before include "Gradual development" , "Darwinian process" , "Gradual change" , "Darwinian theory" , "Darwin's theory" .)