Thin film replaced before sly pupil leaves (6)
I believe the answer is:
flimsy
'thin' is the definition.
('flimsy' can be similar in meaning to 'thin')
'film replaced before sly pupil leaves' is the wordplay.
'replaced' indicates an anagram.
'before' is a charade indicator (letters next to each other).
'pupil' becomes 'l'.
'leaves' indicates named letters should be taken away.
'sly' with 'l' taken away is 'sy'.
'film' with letters rearranged gives 'flim'.
'flim'+'sy'='FLIMSY'
(Other definitions for flimsy that I've seen before include "Weak, insubstantial" , "Lightweight" , "Light and thin, fragile" , "Light and thin, material or argument, perhaps" , "Fragile, of loose structure" .)