Sword unstable peer holds (4)
I believe the answer is:
epee
'sword' is the definition.
(sword used in fencing)
'unstable peer holds' is the wordplay.
'holds' indicates a hidden word.
'EPEE' is hidden within 'unstable peer'.
(Other definitions for epee that I've seen before include "Narrow fencing sword" , "Fencing blade" , "Type of sword" , "Thin fencing sword" , "Fencing weapon" .)