Important sign to American after end of creationism (9)
I believe the answer is:
momentous
'important' is the definition.
('momentous' can be similar in meaning to 'important')
'sign to american after end of creationism' is the wordplay.
'sign' becomes 'omen' (omen is a kind of sign**).
'american' becomes 'us'.
'after' says to put letters next to each other.
'end of' indicates one should take the final letters.
The final letter of 'creationism' is 'm'.
'omen'+'to'+'us'='omentous'
'omentous' put after 'm' is 'MOMENTOUS'.
(Other definitions for momentous that I've seen before include "major" , "of great significance" .)