Wrongly declare one's piece of data in note (8)
I believe the answer is:
misstate
'wrongly declare' is the definition.
I can't judge whether this defines the answer.
'one's piece of data in note' is the wordplay.
'one's' becomes 'i's' (Roman numeral).
'piece of data' becomes 'stat' (short for statistic).
'in' indicates putting letters inside.
'note' becomes 'me' (musical note as in do-re-me).
'is'+'stat'='isstat'
'isstat' going within 'me' is 'MISSTATE'.
Can you help me to learn more?
(Other definitions for misstate that I've seen before include "Wrongly announce" , "Put incorrectly" , "Declare wrongly" , "Mast site (anag.)" , "Inaccurately report" .)