Not a bad go at doing a favour (4,4)
I believe the answer is:
good turn
'a favour' is the definition.
(good turn is a kind of favour)
'not a bad go at doing' is the wordplay.
I cannot really understand how this works, but
'not' could be 'o' (I've seen this in other clues) and 'o' is located in the answer.
'go' could be 'turn' (synonyms) and 'turn' is located in the answer.
The remaining letters 'god' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for good turn that I've seen before include "Helpful act" , "Helpful deed" , "Nice act" , "star performer" , "Favour expected" .)