Catchy song (3)
'catchy song' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for bop that I've seen before include "Dance of the 1950s" , "Dance - hit (informal)" , "Lively dance" , "Form of jazz from the 1940s" , "Dance to pop music" .)
(Other definitions for ditty that I've seen before include "Light-hearted song" , "Simple tune" , "Short simple song" , "Little tune" , "Air" .)