Stacked flower - that's a blow (10)
I believe the answer is:
piledriver
'a blow' is the definition.
I know nothing about this answer so I can't judge whether this works.
'stacked flower' is the wordplay.
'stacked' becomes 'piled' ('pile' can be a synonym of 'stack').
'flower' becomes 'river' (A river flows and is therefore a flower).
'piled'+'river'='PILEDRIVER'
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for piledriver that I've seen before include "Machine for knocking heavy stakes into the ground" , "Machine for driving columns into ground - forceful punch" , "Forceful shot" , "Builder's equipment" , "needed for powerful shot" .)