A large amount (6)
(Other definitions for oodles that I've seen before include "'Lashings, heaps (6)'" , "A great many" , "wealth" , "Very great amount (colloq.)" , "Loosed among lots" .)
(Other definitions for stacks that I've seen before include "Orderly heaps as of hay" , "A lot (of hay?)" , "Large tall chimneys" , "Tons" , "bags?" .)
'a large amount' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for plethora that I've seen before include "Unnecessarily large quantity" , "Profusion" , "Glut, over-abundance" , "More than enough" , "Embarrassment" .)