Item of furniture firm when the top is removed (5)
I believe the answer is:
table
'item of furniture' is the definition.
(table is a type of furniture item)
'firm when the top is removed' is the wordplay.
'firm' becomes 'stable' (similar in meaning).
'when the top is removed' means to remove the first letter.
'stable' with its initial letter taken off is 'TABLE'.
(Other definitions for table that I've seen before include "Array of data" , "Board; data list" , "Index" , "shelf" , "put forward a > schedule" .)