Girl, briefly detained, got the wrong idea (14)
I believe the answer is:
misapprehended
'got the wrong idea' is the definition.
The answer and definition can be both related to thinking as well as being past participle verbs.
Perhaps they are linked in a way I don't understand?
'girl briefly detained' is the wordplay.
I cannot really see how this works, but
'girl' could be 'isa' and 'isa' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Another definition for misapprehended that I've seen is " taken the wrong way".)