It may 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.
Perhaps they are linked in a way I don't understand?
'it may do good' is the wordplay.
I cannot really see how this works, but
'do' is found in the answer.
'good' could be 'g' (abbreviation) and 'g' is located in the answer.
No letters remain.
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 "'You cannot teach an old ... new tricks'" , "Canine animal" , "Follow; pet" , "Cur" , "Worry" .)