Heavy weight is in wood floor (8)
I believe the answer is:
astonish
'floor' is the definition.
(I've seen this in another clue)
'heavy weight is in wood' is the wordplay.
'heavy weight' becomes 'ton' (a ton weighs a lot).
'in' means one lot of letters goes inside another.
'wood' becomes 'ash' (wood from an ash tree).
'ton'+'is'='tonis'
'tonis' inserted inside 'ash' is 'ASTONISH'.
(Other definitions for astonish that I've seen before include "Stun" , "are staggering" , "Startle" , "Floor" , "Gobsmack" .)