Grandmother and grandfather, possibly (6)

I believe the answer is:

clocks

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'grandmother' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for clocks that I've seen before include "Timepieces" , "Timers" , "Notices" .)

I've seen this clue in the Evening Standard.

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