Emperor who had to break off with the Americans? (7)

I believe the answer is:

severus

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'emperor' is the definition.
(I know that Severus has the occupation emperor)

'break off with the americans?' is the wordplay.
'break off' becomes 'sever' (I've seen this before).
'with' means one lot of letters go next to another.
'the americans?' becomes 'US' (United States).
'sever'+'us'='SEVERUS'

'who had to' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.

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I've seen this clue in the Evening Standard.

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