Pupil, feeding, asked for knife (5)
I believe the answer is:
blade
'knife' is the definition.
(I've seen this before)
'pupil feeding asked' is the wordplay.
'pupil' becomes 'l'.
'feeding' is an insertion indicator.
'asked' becomes 'bade'.
'l' put within 'bade' is 'BLADE'.
'for' acts as a link.
(Other definitions for blade that I've seen before include "Cutting edge" , "Flat part of an oar" , "See 24 Across" , "Cutting part of grass?" , "Runner on an ice-skate" .)