Re: Russia: I left, all upset - he provided cover (7)

I believe the answer is:

assurer

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'all upset he provided cover' is the definition.
The definition suggests a singular noun which matches the answer.

're russia i left' is the wordplay.
'russia' becomes 'russ' (I can't explain this - if you can you should give a lot more credence to this answer).
'i' becomes 'a'.
'left' is a reversal indicator.
're'+'russ'+'a'='rerussa'
'rerussa' back-to-front is 'ASSURER'.

Can you help me to learn more?

(Other definitions for assurer that I've seen before include "One providing insurance" , "One promising" , "I offer back-up" , "Underwriter" .)

I've seen this clue in The Independent.

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