Keep about fifty to work as drudges (5)

I believe the answer is:

slave

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'work as drudges' is the definition.
I can't tell whether this definition defines the answer.

'keep about fifty' is the wordplay.
'keep' becomes 'save' (synonyms).
'about' indicates putting letters inside.
'fifty' becomes 'L' (Roman numeral).
'save' placed around 'l' is 'SLAVE'.

'to' acts as a link.

Can you help me to learn more?

(Other definitions for slave that I've seen before include "Unpaid worker" , "Person owned by another" , "Person who is the property of another" , "Person who is owned by someone" , "Person in thraldom" .)

I've seen this clue in The Irish News.

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