Mind Celt let in mistakenly (9)
I believe the answer is:
intellect
'mind' is the definition.
('intellect' can be a synonym of 'mind')
'celt let in mistakenly' is the wordplay.
'mistakenly' is an anagram indicator.
'celt'+'let'+'in'='celtletin'
'celtletin' anagrammed gives 'INTELLECT'.
(Other definitions for intellect that I've seen before include "Person with brilliant mind" , "understanding" , "Brain" , "Reason" , "Cognitive faculty" .)