Crossword Genius

Not very good doctor about to introduce oxygen (8)

Ross

I believe the answer is:

mediocre

'not very good' is the definition.
(mediocre means of middling quality)

I don't understand the rest of the clue.

(Other definitions for mediocre that I've seen before include "Second-rate, undistinguished" , "Fair, middle of the road" , "Rather poor" , "Come ride (anag.)" , "Rather inferior" .)

I've seen this clue in The Times.
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