American firm takes on middle section of seventeenth floor (7)
I believe the answer is:
astound
'floor' is the definition.
(I've seen this in another clue)
'american firm takes on middle section of seventeenth' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'firm takes' becomes 'sound' (I can't explain this - if you can you should give a lot more credence to this answer).
'on' means one lot of letters goes inside another (as in clothing 'on' a person).
'middle section of' says to take the centre.
The middle letter of 'seventeenth' is 't'.
'sound' going around 't' is 'stound'.
'a'+'stound'='ASTOUND'
Can you help me to learn more?
(Other definitions for astound that I've seen before include "Startle" , "Stagger" , "Bowl over" , "Bewilder" , "Flabbergast" .)