Composer's pen (4)

I believe the answer is:

cage

I believe this is a double definition.

'composer's' is the first definition.
(I've seen this before)

'pen' is the second definition.
(I've seen this before)

(Other definitions for cage that I've seen before include "Enclosure made from metal bars" , "Place of confinement" , "In places squirrel's nest" , "Nicolas - - (films)" , "Confine -- US composer, d. 1992" .)

I've seen this clue in The Sun.

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