Filter tricky question about school (3-8)
I believe the answer is:
tea-strainer
'filter' is the definition.
(tea-strainer is a kind of filter)
'tricky question about school' is the wordplay.
'tricky question' becomes 'teaser' (I've seen this in another clue).
'about' is an insertion indicator.
'school' becomes 'train' (training is a kind of schooling).
'teaser' enclosing 'train' is 'TEA-STRAINER'.
(Other definitions for tea-strainer that I've seen before include "What stops leaves getting through" , "Catcher of leaves" .)