Sing an opportune rhyme (5)
I believe the answer is:
croon
'sing' is the definition.
(crooning is a kind of singing)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for croon that I've seen before include "Sing softly" , "Sing with a soft voice" , "Sing in a soft low tone" , "Sing like Bing" .)