Observe with sly glance on turning (4)

I believe the answer is:

keep

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'observe' is the definition.
('keep' can be a synonym of 'observe')

I don't understand the rest of the clue.

(Other definitions for keep that I've seen before include "Retain possession of" , "Hold - part of castle" , "Save" , "Retain or care for" , "Continue" .)

I've seen this clue in The Telegraph.

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