Do good, good dog (7)
I believe the answer is:
pigtail
'do' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'good good dog' is the wordplay.
'good' becomes 'pi' (old-fashioned term for pious).
'good' becomes 'g' (abbreviation).
'dog' becomes 'tail' (synonyms).
'pi'+'g'+'tail'='PIGTAIL'
Can you help me to learn more?
(Other definitions for pigtail that I've seen before include "Plait of hair at back of head" , "Braid of hair" , "Schoolgirl's plait" , "Queue of hair" , "Girl's hair style" .)