Sweet, immortal queen OK (11)
I believe the answer is:
marshmallow
'sweet' is the definition.
(I know that marshmallow is a type of sweet)
'immortal queen ok' is the wordplay.
'immortal' becomes 'Mars' (Mars was the Roman god of war).
'queen' becomes 'hm' (short for Her Majesty).
'ok' becomes 'allow' (I've seen this in another clue).
'mars'+'hm'+'allow'='MARSHMALLOW'
(Other definitions for marshmallow that I've seen before include "Soft sweet" , "Spongy confection - pink-flowering plant" , "Spongy jellylike sweet" , "sweet little thing" , "Sweet, often toasted" .)