Flood defence breached by god with hammer (8)
I believe the answer is:
plethora
'flood' is the definition.
(I've seen this before)
'defence breached by god with hammer' is the wordplay.
'defence' becomes 'plea' (a plea is a legal defence).
'breached by' indicates putting letters inside (inserted letters break into or breach the word).
'god with hammer' becomes 'Thor' (Norse god of thunder).
'plea' placed around 'thor' is 'PLETHORA'.
(Other definitions for plethora that I've seen before include "Over-fullness, excess" , "opposite of scarcity" , "Superabundance" , "flood" , "'Large, perhaps excessive, amount (8)'" .)