Train monkey abandoned by staff (5)

I believe the answer is:

drill

I'm a little stuck... Click here to teach me more about this clue!

This clue could be a double definition.

'train' is the first definition.
(drilling is a kind of training)

'monkey abandoned by staff' is the second definition.
'drill' can be an answer for 'monkey' (I've seen this before). I am not sure about the 'abandoned by staff' bit.

Can you help me to learn more?

(Other definitions for drill that I've seen before include "Tool; military training" , "Military training; bore" , "Twill-weave cotton cloth" , "Routine" , "Procedure - practice" .)

I've seen this clue in The Guardian.

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