19 across
INXS
provided
Yates’s
first
shoot
up
(9)
INXS provided Yates’s first shoot up (9)
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The definition is at the end of the clue.
How the clue works
Look at shoot up at the end — that's your definition, meaning to escalate or intensify. For the wordplay, take INXS and read it letter by letter: I, N, then X which gives you TEN as a Roman numeral, then S — so you've got INTENS. After that, provided gives you IF, and Yates's first plucks out the opening letter Y, and string them all together and there it is.