Floated on top of putrid bog (5)
I believe the answer is:
swamp
'bog' is the definition.
(I've seen this before)
'floated on top of putrid' is the wordplay.
'floated' becomes 'swam' ('swim' can be a synonym of 'float').
'on top of' says to take the initial letters (I've seen 'on top' mean this).
The initial letter of 'putrid' is 'p'.
'swam'+'p'='SWAMP'
(Other definitions for swamp that I've seen before include "Overwhelm; wetland" , "Mire" , "Seasonally flooded low land" , "Slough" , "marshy tract" .)