Crossword Genius

It contains shifting sand over time (9)

Ross

I believe the answer is:

hourglass

'it contains shifting sand over' is the definition.
The definition suggests a singular noun which matches the answer.

'time' is the wordplay.
I cannot really see how this works, but
'time' could be 'hour' (hour is a kind of time) and 'hour' is present in the answer.
The remaining letters 'glass' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

(Other definitions for hourglass that I've seen before include "shows the passing of time" , "classic figure" , "Simple timer" , "Sand timer" , "old timer" .)

I've seen this clue in The Independent.
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