Entertaining American soldier's growths (5)
I believe the answer is:
fungi
'growths' is the definition.
'entertaining american soldier's' is the wordplay.
'entertaining' becomes 'fun' (similar in meaning).
'american soldier' becomes 'GI' (informal term for a US soldier).
'fun'+'gi'='FUNGI'
(Other definitions for fungi that I've seen before include "Spore-producing organisms" , "life in woodland?" , "Spore producers" , "Group with some killers" , "woodland feature" .)