American involved in jolly stuff (3)
I believe the answer is:
ram
'stuff' is the definition.
(ram can mean to stuff or push)
'american involved in jolly' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'involved in' indicates putting letters inside.
'jolly' becomes 'RM' (jolly can mean a Royal Marine).
'a' inserted inside 'rm' is 'RAM'.
(Other definitions for ram that I've seen before include "'Computer memory, initially (3)'" , "Tup" , "The constellation Aries" , "Crash into, maybe for battering" , "Deliberately collide with" .)