Cello-playing involves one dog (6)
I believe the answer is:
collie
'dog' is the definition.
(collie is a kind of dog)
'cello-playing involves one' is the wordplay.
'playing' indicates anagramming the letters.
'involves' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'cello' anagrammed gives 'colle'.
'colle' placed around 'i' is 'COLLIE'.
(Other definitions for collie that I've seen before include "Long-haired sheepdog" , "Working animal" , "See 2" , "Farm dog" , "Shepherd's dog" .)