Calm one among many quite agitated (10)
I believe the answer is:
equanimity
'calm' is the definition.
(I know this)
'one among many quite agitated' is the wordplay.
'one' becomes 'i' (Roman numeral).
'among' indicates putting letters inside.
'agitated' indicates anagramming the letters.
'many'+'quite'='manyquite'
'manyquite' with letters rearranged gives 'equanimty'.
'i' inserted within 'equanimty' is 'EQUANIMITY'.
(Other definitions for equanimity that I've seen before include "Composure" , "Sangfroid" .)