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 'I'll' (abbreviation).
'be behind' says to put letters next to each other.
'small' becomes 's' (abbreviation - e.g. clothes size).
'ill' put 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" .)