Iris turns on American star (6)
I believe the answer is:
sirius
'star' is the definition.
(Sirius is an example)
'iris turns on american' is the wordplay.
'turns' says the letters should be written in reverse.
'on' is a charade indicator (letters next to each other).
'american' becomes 'us' (abbreviation for United States).
'iris' back-to-front is 'siri'.
'siri'+'us'='SIRIUS'
(Other definitions for sirius that I've seen before include "The Dog Star that follows the hunter Orion" , "Brightest star in the sky" , "The Dog Star, brightest of the fixed stars" , "Prominent star" , "Bright star" .)