Significant American in pursuit of second Oscar (9)
I believe the answer is:
momentous
'significant' is the definition.
(similar in meaning)
'american in pursuit of second oscar' is the wordplay.
'american' becomes 'us' (abbreviation for United States).
'in pursuit of' is a charade indicator (letters next to each other).
'second' becomes 'moment' (synonyms).
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'moment'+'o'='momento'
'us' after 'momento' is 'MOMENTOUS'.
(Other definitions for momentous that I've seen before include "of great significance" , "major" .)