Bit of a do-gooder, if you follow (3)

I believe the answer is:

dog

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'follow' is the definition.
(I know that dog is a type of follow)

I cannot understand how the rest of the clue works.

(Other definitions for dog that I've seen before include "Cur" , "Hound" , "St Bernard, for example" , "'You cannot teach an old ... new tricks'" , "Contemptible man" .)

I've seen this clue in the Evening Standard.

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