Unintended stress to a learner I'd kept in (10)
I believe the answer is:
accidental
'unintended' is the definition.
('accidental' can be similar in meaning to 'unintended')
'stress to a learner i'd kept in' is the wordplay.
'stress' becomes 'accent' ('accent' can be a synonym of 'stress').
'learner' becomes 'l'.
'kept in' is an insertion indicator.
'accent'+'a'+'l'='accental'
'accental' enclosing 'id' is 'ACCIDENTAL'.
(Other definitions for accidental that I've seen before include "indication to modify tone slightly" , "Possible sharp" , "Unintended - sort of note" , "change of key" , "without planning" .)