"Unforgettable" singer (4)
I believe the answer is:
cole
'unforgettable singer' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for cole that I've seen before include "Old nursery rhyme king" , "Elderly monarch" , "died in 2015" , "Old English king!" , "Cabbage" .)