Unexpected role Garland’s taken on — a siren (7)
I believe the answer is:
lorelei
'siren' is the definition.
(I know that Lorelei is a siren)
'unexpected role garland's' is the wordplay.
'unexpected' indicates anagramming the letters.
'garland' becomes 'lei' (a lei is a garland of flowers).
'role' with letters rearranged gives 'lore'.
'lore'+'lei'='LORELEI'
'taken on a' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for lorelei that I've seen before include "Temptress" , "Famous Rhine rock" , "Dangerous female" , "Enchanting singer" , "High rock in the Rhine gorge" .)