25 across
Thumps
one
with
Guardian
heartlessly
–
it’s
a
mark
of
the
Greens?
(5,4)
Thumps one with Guardian heartlessly – it’s a mark of the Greens? (5,4)
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The definition is at the end of the clue.
How the clue works
So mark of the Greens is a clever way of saying the £ symbol — "Greens" here is slang for money. You get POUNDS from "thumps", then I for "one", then take the heart out of "Guardian" to leave just G and N. String them all together and there's your answer.