Swamp Commons with Liberal intake (5)
I believe the answer is:
flood
'swamp' is the definition.
(I've seen this before)
'commons with liberal intake' is the wordplay.
'commons' becomes 'food' (commons could historically mean food).
'with' means one lot of letters goes inside another.
'liberal intake' becomes 'l' (I can't justify this - if you can you should believe this answer much more).
'food' going around 'l' is 'FLOOD'.
(Other definitions for flood that I've seen before include "Inundate" , "Oversupply" , "lot of water" , "Light" , "Overwhelming amount" .)