Poorly behind small ledge (4)
I believe the answer is:
sill
'ledge' is the definition.
(I've seen this in another clue)
'poorly behind small' is the wordplay.
'poorly' becomes 'ill' (similar in meaning).
'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 "Shelf at the foot of a window" , "Underwater rock ledge" , "Mass of rock" , "Ledge beneath a window" , "Horizontal intrusion of igneous rock between sedimentaries, as in much of Hadrian's Wall" .)