Prepare to fire stone in battleground (7)
I believe the answer is:
cockpit
'battleground' is the definition.
(place for cockfights)
'prepare to fire stone' is the wordplay.
'prepare to fire' becomes 'cock' (as in cocking a gun).
'stone' becomes 'pit' (synonyms).
'cock'+'pit'='COCKPIT'
'in' is the link.
(Other definitions for cockpit that I've seen before include "In a plane, the pilot sits in this space" , "Place where there was fighting" , "Site of numerous battles or campaigns" , "Compartment for aircraft pilot" , "Place for aircraft's pilot and crew" .)