Substantial weight is in wood floor (8)
I believe the answer is:
astonish
'floor' is the definition.
(thesaurus)
'substantial weight is in wood' is the wordplay.
'substantial weight' becomes 'ton' (ton is a kind of weight. I am not sure about the 'substantial' bit.**).
'in' is an insertion indicator.
'wood' becomes 'ash' (I've seen this before).
'ton'+'is'='tonis'
'tonis' put into 'ash' is 'ASTONISH'.
Can you help me to learn more?
(Other definitions for astonish that I've seen before include "Stun" , "Greatly surprise, amaze" , "Startle" , "Gobsmack" , "Take by surprise" .)