A soldier in the pampas? That’s bananas (9)

'bananas' is the definition.
(plantain is a kind of banana)

'a soldier in the pampas?' is the wordplay.
'a soldier' becomes 'ant' (I've seen this before).
'in' indicates putting letters inside.
'the pampas?' becomes 'plains' (The Pampas are a series of South American plains).
'ant' going within 'plains' is 'PLANTAINS'.

'that's' is the link.