Detective mixing up us with the student found inside (6)
I believe the answer is:
sleuth
'detective' is the definition.
(I know that sleuth is a type of detective)
'mixing up us with the student found inside' is the wordplay.
'mixing up' indicates anagramming the letters.
'with' is a charade indicator (letters next to each other).
'student' becomes 'L' (as in L-plates for learner drivers).
'found inside' is an insertion indicator.
'us' after 'the' is 'theus'.
'theus' is an anagram of 'seuth'.
'seuth' going around 'l' is 'SLEUTH'.
(Other definitions for sleuth that I've seen before include "Detective who follows a trail" , "Gumshoe" , "Hustle for a clever detective like Sherlock" , "A detective like Sherlock - given to hustle" , "Detective; play and film" .)