Something up your sleeve (6)
I believe the answer is:
biceps
'something up your sleeve' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for biceps that I've seen before include "Weightlifters use them" , "Contractor" , "Forearm-flexing muscle" , "Arm muscles" , "Muscle that flexes the forearm" .)