Flood defence breached by Norse god (8)
I believe the answer is:
plethora
'flood' is the definition.
(I've seen this before)
'defence breached by norse god' is the wordplay.
'defence' becomes 'plea' (plea is a kind of defence).
'breached by' indicates putting letters inside.
'norse god' becomes 'thor' (Thor is an example).
'plea' placed around 'thor' is 'PLETHORA'.
(Other definitions for plethora that I've seen before include "Excessive quantity" , "Abundance" , "Oversupply" , "Excess, glut" , "almost too much" .)