Bit of timber in a solid floor (7)
I believe the answer is:
astound
'floor' is the definition.
(I've seen this before)
'bit of timber in a solid' is the wordplay.
'bit of' says to take the initial letters.
'in' means one lot of letters goes inside another.
'solid' becomes 'sound' (similar in meaning).
The first letter of 'timber' is 't'.
'a'+'sound'='asound'
't' put inside 'asound' is 'ASTOUND'.
(Other definitions for astound that I've seen before include "Bewilder" , "Stupefy" , "Shock - amaze" , "Stagger" , "Fill with wonder or astonishment" .)