A well-behaved dog carrying the ball back (4)
I believe the answer is:
good
'a well-behaved' is the definition.
(I've seen this in another clue)
'dog carrying the ball back' is the wordplay.
'carrying' means one lot of letters goes inside another.
'the ball' becomes 'o' (O looks like a ball).
'back' says the letters should be written backwards.
'dog' enclosing 'o' is 'doog'.
'doog' reversed gives 'GOOD'.
(Other definitions for good that I've seen before include "Well-behaved, kind, or virtuous" , "Carol character from 1 as of old" , "Obedient" , "Virtuous - valid" , "Of a high standard" .)