Mushrooms giving amusement to American soldier (6)

I believe the answer is:

fungi

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But I can't explain it at all! Could you help me understand?

(Other definitions for fungi that I've seen before include "Yeast, etc" , "Plants like mushrooms and toadstools" , "Group with some killers" , "Yeasts, moulds, mushrooms etc" , "Magic mushrooms perhaps" .)

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