Top firm? Not so (3)
I believe the answer is:
lid
'top' is the definition.
(lid is a kind of top)
'firm? not so' is the wordplay.
'firm?' becomes 'solid' ('solid' can be a synonym of 'firm').
'not' indicates named letters should be taken away.
'solid' with 'so' removed is 'LID'.
(Other definitions for lid that I've seen before include "Receptacle cover" , "Kind of cover, may be hinged" , "Hinged or removable cover as for bottle" , "Removable cover of a container" , "Cover as for jar" .)