The most common and most likely answer to this crossword clue is the 7 letter word DIRTIED
I've seen this crossword clue in The Times
D
I
R
T
I
E
D
Buddy explains!

The straight part
We know that "contaminated" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case contaminated = dirtied.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does DIRTIED mean?
DIRTIED verb- To make (something) dirty.
- To stain or tarnish (somebody) with dishonor.
- To debase by distorting the real nature of (something).
- To become soiled.