Most of planet consumed in flood (8)
I believe the answer is:
saturate
'flood' is the definition.
(thesaurus)
'most of planet consumed' is the wordplay.
'most of' means to remove the last letter.
'planet' becomes 'saturn' (Saturn is an example).
'consumed' becomes 'ate' (eating is a kind of consuming).
'saturn' with its last letter taken off is 'satur'.
'satur'+'ate'='SATURATE'
'in' acts as a link.
(Other definitions for saturate that I've seen before include "Infuse or fill completely" , "Make thoroughly wet, soak" , "steep" , "Use attar to make thoroughly wet" , "Oversupply" .)