America acquires first instalment of Rupert Bear (4)
I believe the answer is:
ursa
'bear' is the definition.
(Latin name for bear as in Ursa Major)
'america acquires first instalment of rupert' is the wordplay.
'america' becomes 'usa' ('U.S.A.' can be a synonym of 'america').
'acquires' indicates putting letters inside.
'first instalment of' suggests taking the first letters.
The initial letter of 'rupert' is 'r'.
'usa' placed around 'r' is 'URSA'.
(Other definitions for ursa that I've seen before include "Major/Minor constellation" , "'Major and Minor, constellations (4)'" , "Latin for the Great Bear, . . . . Major" , "and 18 Down: The Great Bear" , "- Major, - Minor, prominent constellations" .)