Colourful record covers for an old song (12)
I believe the answer is:
'song' is the definition.
(I've seen this before)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for greensleeves that I've seen before include "that might be noticed when lying on the lawn" , "air of the past" , "Brownshirts would never display such" , "English folk song (not composed by Henry VIII!)" , "Air essentially unchanged for centuries!?" .)