(Of tune) memorable (6)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
catchy
(Other definitions for catchy that I've seen before include "(Of a tune) instantly appealing and memorable" , "Hummable" , "Of a tune, lively and easy to remember" , "Readily caught, like a tune" , "(Of tune) infectious" .)