Chant a simple ditty (9)
I believe the answer is:
plainsong
'chant' is the definition.
(plainsong is a kind of chant)
'a simple ditty' is the wordplay.
'a simple' becomes 'plain' (I've seen this before).
'ditty' becomes 'song' (I've seen this before).
'plain'+'song'='PLAINSONG'
(Other definitions for plainsong that I've seen before include "Lied, perhaps" , "sacred music" , "Medieval style of church music" , "Simple melody" , "church melody" .)