Mushrooms amusing American soldier (5)
I believe the answer is:
fungi
'mushrooms' is the definition.
(I've seen this before)
'amusing american soldier' is the wordplay.
'amusing' becomes 'fun' (synonyms).
'american soldier' becomes 'GI' (informal term for a US soldier).
'fun'+'gi'='FUNGI'
(Other definitions for fungi that I've seen before include "Group with some killers" , "Eg, toadstools" , "woodland feature" , "Toadstools, say" , "Yeasts, moulds, mushrooms etc" .)