In sum, the gutless prime minister's a relative newcomer (7)

I believe the answer is:

stepmum

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'a relative newcomer' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.

'in sum the gutless prime minister's' is the wordplay.
'in' indicates putting letters inside.
'gutless' suggests removing the centre.
'prime minister' becomes 'PM' (abbreviation).
'the' with its centre taken out is 'te'.
'te'+'pm'='tepm'
'sum' placed around 'tepm' is 'STEPMUM'.

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I've seen this clue in The Independent.

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