Sleep? Perchance, except those seen in schools in dorm, oddly (6)
I believe the answer is:
dancer
'sleep?' is the definition.
(Wayne Sleep is an example)
'perchance except those seen in schools in dorm oddly' is the wordplay.
'except' is a deletion indicator.
'those seen in schools' becomes 'perch' (school can mean a group of perch fish).
'in' means one lot of letters goes inside another.
'dorm oddly' becomes 'dr' (I can't explain this - if you can you should give a lot more credence to this answer).
'perchance' with 'perch' removed is 'ance'.
'ance' inserted into 'dr' is 'DANCER'.
(Other definitions for dancer that I've seen before include "Nureyev?" , "Mover to music" , "Craned (anag.)" , "Eg, ballerina" , "Terpsichorean" .)