Old Bob is not doing well under Casement (4)
I believe the answer is:
sill
'casement' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe you can see a link between them that I can't see?
'old bob is not doing well under' is the wordplay.
'old bob' becomes 's' (abbreviation for bob or shilling used before i97i).
'is not doing well' becomes 'ill' (I can't explain this - if you can you should believe this answer much more).
'under' is a charade indicator (letters next to each other).
's'+'ill'='SILL'
Can you help me to learn more?
(Other definitions for sill that I've seen before include "Slab at foot of window" , "Ledge beneath a window" , "Window edge" , "Shelf at the foot of a window" , "Mass of rock" .)