Floated south across country with duck departing marshes (10)

'marshes' is the definition.
(I've seen this before)

'floated south across country' is the wordplay.
'floated' becomes 'swam' ('swim' can be a synonym of 'float').
'south' becomes 'ps' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'across' indicates putting letters inside.
'country' becomes 'land' (synonyms).
'ps' placed around 'land' is 'plands'.
'swam'+'plands'='SWAMPLANDS'

'with duck departing' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.