Endlessly stupid place for a window box? (4)
I believe the answer is:
sill
'place for a window box?' is the definition.
(I've seen this in another clue)
'endlessly stupid' is the wordplay.
'endlessly' means to remove the last letter.
'stupid' becomes 'silly' (I've seen this before).
'silly' with its final letter removed is 'SILL'.
(Other definitions for sill that I've seen before include "Slab at foot of window" , "Horizontal timber forming the bottom of a framework" , "Shelf at the foot of a window" , "Underwater rock ledge" , "Mass of rock" .)