Second American keeps ring for major (9)
I believe the answer is:
momentous
'major' is the definition.
(I've seen this before)
'second american keeps ring' is the wordplay.
'second' becomes 'moment' (synonyms).
'american' becomes 'us' (abbreviation for United States).
'keeps' means one lot of letters goes inside another.
'ring' becomes 'o' (letter resembling a ring).
'moment'+'us'='momentus'
'momentus' enclosing 'o' is 'MOMENTOUS'.
'for' is the link.
(Other definitions for momentous that I've seen before include "of great significance" , "major" .)