Did butterfly possibly start to pollute wetland? (5)
I believe the answer is:
Here is my best explanation:
'wetland?' is the definition.
'did butterfly possibly start to pollute' is the wordplay.
'did butterfly possibly' becomes 'swam' (butterfly is a type of swimming stroke).
'start to' suggests taking the first letters.
The initial letter of 'pollute' is 'p'.
(Other definitions for swamp that I've seen before include "Mire", "Boggy land", "Quagmire; inundate", "Bog, marsh", "marshy tract", "Waterlogged ground", "In flood", "Seasonally-flooded lowland", "Seasonally flooded lowland" and "Inundate".)