I will be behind small ledge (4)
I believe the answer is:
sill
'ledge' is the definition.
(I've seen this before)
'i will be behind small' is the wordplay.
'i will' becomes 'I'll' (abbreviation).
'be behind' is a charade indicator (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 "Window edge" , "Slab at foot of window" , "Mass of rock" , "Ledge beneath a window" , "Underwater rock ledge" .)