It could do good as a guide (3)
I believe the answer is:
dog
'a guide' is the definition.
The answer and definition can be both people as well as being singular nouns.
Maybe you can see a link between them that I can't see?
'it could do good' is the wordplay.
I cannot quite see how this works, but
'do' is present in the answer.
'good' could be 'g' (abbreviation) and 'g' is found in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'as' is the link.
Can you help me to learn more?
(Other definitions for dog that I've seen before include "Hound" , "Worry" , "St Bernard, for example" , "Domesticated canine" , "Cur" .)