Something up one's sleeve? (3)
I believe the answer is:
arm
'something up one's sleeve?' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for arm that I've seen before include "Human limb" , "Test of strength with elbows on the table" , "What soldiers do" , "ready for war" , "Upper limb" .)