I will be behind small shelf (4)
I believe the answer is:
sill
'shelf' is the definition.
(I've seen this before)
'i will be behind small' is the wordplay.
'i will' becomes 'ill' (I've seen this before).
'be behind' is a charade indicator (letters next to each other).
'small' becomes 's' (abbreviation - e.g. clothes size).
'ill' after 's' is 'SILL'.
(Other definitions for sill that I've seen before include "Horizontal timber forming the bottom of a framework" , "Shelf at the foot of a window" , "Ledge beneath a window" , "Window edge" , "Mass of rock" .)