Stallone's back training with Eastern weapon (4)
I believe the answer is:
epee
'weapon' is the definition.
(epee is a kind of weapon)
'stallone's back training with eastern' is the wordplay.
'back' indicates one should take the final letters (the letter at the back).
'training' becomes 'PE' (short for Physical Education).
'with' means one lot of letters go next to another.
'eastern' becomes 'e'.
The final letter of 'stallone' is 'e'.
'e'+'pe'+'e'='EPEE'
(Other definitions for epee that I've seen before include "Type of sword" , "sports equipment" , "Sharp-pointed duelling sword" , "Fencing sword similar to a foil" , "Thin fencing sword" .)