Crossword Genius

I had to abandon grubbier friend (6)

Ross

I believe the answer is:

mucker

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'friend' is the definition.
(affectionate term, as in 'my old mucker')

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for mucker that I've seen before include "Mate - shifter of rubbish" , "Friend (in a mess?)" .)

I've seen this clue in The Irish Times.
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