After school I will get shelf (4)
I believe the answer is:
sill
'shelf' is the definition.
(I've seen this before)
'after school i will' is the wordplay.
'after' means one lot of letters go next to another.
'school' becomes 'S'.
'i will' becomes 'I'll' (abbreviation).
's' next to 'ill' is 'SILL'.
'get' acts as a link.
(Other definitions for sill that I've seen before include "Slab at foot of window" , "Horizontal timber forming the bottom of a framework" , "Horizontal intrusion of igneous rock between sedimentaries, as in much of Hadrian's Wall" , "Window edge" , "Mass of rock" .)