Crossword Genius

Doctor, say, has small bits left (5)

Ross

I believe the answer is:

dregs

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'bits left' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

'doctor say has small' is the wordplay.
'doctor' becomes 'dr' (abbreviation).
'say' becomes 'eg' (both can mean for example).
'has' is a charade indicator (letters next to each other).
'small' becomes 's' (abbreviation - e.g. clothes size).
'dr'+'eg'+'s'='DREGS'

Can you help me to learn more?

(Other definitions for dregs that I've seen before include "Wine or coffee residue" , "Lees - riffraff" , "Residue, lees" , "Most worthless part (of anything)" , "Sediment at bottom of drink" .)

I've seen this clue in The Mirror.
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