Yet to remove second shelf (4)
I believe the answer is:
sill
'shelf' is the definition.
(I've seen this before)
'yet to remove second' is the wordplay.
I cannot really see how this works, but
'second' could be 's' (common abbreviation) and 's' is present in the answer.
The remaining letters 'ill' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(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" , "Window edge" , "Mass of rock" .)