Retiring professor gets to come over boorish (5)

I believe the answer is:

gross

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'retiring' is the definition.
Both the answer and definition are adjectives. Perhaps they are linked in a way I don't understand?

'professor gets to come over boorish' is the wordplay.
'GROSS' is hidden back-to-front within the letters.
However, I'm not sure how this is indicated.

Can you help me to learn more?

(Other definitions for gross that I've seen before include "clearly wrong" , "Repulsively large" , "Description of crude" , "one-hundred and fourty-four" , "Arrant" .)

I've seen this clue in The Telegraph.

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