Mistakenly declare acquitted (7)
I believe the answer is:
cleared
'acquitted' is the definition.
('clear' can be a synonym of 'acquit')
'mistakenly declare' is the wordplay.
'mistakenly' indicates anagramming the letters.
'declare' anagrammed gives 'CLEARED'.
(Other definitions for cleared that I've seen before include "Profited by" , "Jumped over" , "Exonerated" , "approved" , "Brightened up" .)