Highest and lowest function values are met with ten changes (7)

I believe the answer is:

extrema

'highest and lowest function values' is the definition.
(plural of 'extremum')

'are met with ten changes' is the wordplay.
'with' is a charade indicator (letters next to each other).
'ten' becomes 'x' ('X' can be a synonym of 'ten').
'changes' indicates anagramming the letters.
'are'+'met'+'x'='aremetx'
'aremetx' with letters rearranged gives 'EXTREMA'.

(Another definition for extrema that I've seen is " Furthest points".)

I've seen this clue in The Independent.

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