Watch heard thus as mattress cover (7)

I believe the answer is:

ticking

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'mattress cover' is the definition.
(I've seen this before)

I can't explain the rest of the clue.

(Other definitions for ticking that I've seen before include "What may be under sheet" , "Evidence of clock going" , "Mattress cover" , "Material; sounding like a clock" , "Mattress textile" .)

I've seen this clue in the Sydney Morning Herald.

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