Crossword Genius

Musical instrument possibly containing oil? (4)

Ross

I believe the answer is:

drum

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

'musical instrument' is the definition.
(I know that Drum is a musical instrument)

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

Can you help me to learn more?

(Other definitions for drum that I've seen before include "12th" , "beat" , "See 18" , "Percussion instrument" , "Spool; beaten instrument" .)

I've seen this clue in the Evening Standard.
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