Stern former ruler I am to follow (4)

I believe the answer is:

grim

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'stern' is the definition.
(I know that stern can be written as grim)

'former ruler i am to follow' is the wordplay.
'former ruler' becomes 'gr' (short for Georgius Rex, King George in Latin. I am not sure about the 'former' bit.).
'i am' becomes 'I'm' (contraction).
'to follow' says to put letters next to each other (some letters go after or follow others).
'gr'+'im'='GRIM'

Can you help me to learn more?

(Other definitions for grim that I've seen before include "(Of humour) black; unappealing, depressing" , "Harsh and merciless" , "Severe; horrible" , "looking black" , "Stern and unsmiling" .)

I've seen this clue in The Times.

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