Opposed tons being dumped in an island (4)
I believe the answer is:
anti
'opposed' is the definition.
(similar in meaning)
'tons being dumped in an island' is the wordplay.
'tons' becomes 't'.
'being dumped in' means one lot of letters goes inside another.
'island' becomes 'i'.
'an'+'i'='ani'
't' placed inside 'ani' is 'ANTI'.
(Other definitions for anti that I've seen before include "(Person who is) not in favour" , "one opposing?" , "Policy's opponent" , "'No' voter" , "not favoured" .)