American soldier visiting upset Christmas host (6)
I believe the answer is:
legion
'host' is the definition.
('legion' can be a synonym of 'host')
'american soldier visiting upset christmas' is the wordplay.
'american soldier' becomes 'gi'.
'visiting' is an insertion indicator.
'upset' is a reversal indicator.
'christmas' becomes 'noel' ('Noel' can be a synonym of 'christmas').
'noel' written backwards gives 'leon'.
'gi' inserted into 'leon' is 'LEGION'.
(Other definitions for legion that I've seen before include "Roman army grouping" , "Military unit of ancient Rome" , "Roman battalion" , "Large unit of the old Roman army" , "The French have a Foreign one" .)