Did butterfly possibly start to pollute wetland? (5)
I believe the answer is:
swamp
'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'.
'swam'+'p'='SWAMP'
(Other definitions for swamp that I've seen before include "marshy tract" , "Bog, marsh" , "Bog; inundate" , "Marshy land" , "Overwhelm; wetland" .)