First agree about its being in keeping with (10)

I believe the answer is:

consistent

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'its being in keeping with' is the definition.
I can't tell whether this definition defines the answer.

'first agree about' is the wordplay.
'first' becomes 'ist'.
'agree' becomes 'consent' (I've seen this before**).
'about' means one lot of letters goes inside another.
'ist' going into 'consent' is 'CONSISTENT'.

Can you help me to learn more?

(Other definitions for consistent that I've seen before include "compatible" , "true to priniciples" , "uniform" , "stable" , "Unchanging in standard over time" .)

I've seen this clue in the Evening Standard.

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