Fill the spaces in a working routine (5)

I believe the answer is:

grout

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'fill the spaces' is the definition.
The answer and definition can be both substances as well as being singular nouns.
Maybe there's an association between them I don't understand?

'in a working routine' is the wordplay.
'in' indicates a hidden word.
'GROUT' is hidden in the letters of 'working routine'.

Can you help me to learn more?

(Other definitions for grout that I've seen before include "Thin, fluid mortar" , "Grub" , "Thin mortar for filling cracks" , "cement" , "Tile mortar" .)

I've seen this clue in the Evening Standard.

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