Set pieces oddly missing fencing kit (4)
I believe the answer is:
epee
'fencing kit' is the definition.
(sword used in fencing)
'set pieces oddly missing' is the wordplay.
'oddly missing' indicates alternate letters.
'set'+'pieces'='setpieces'
The alternate letters of 'setpieces' are 'EPEE'.
(Other definitions for epee that I've seen before include "Rapier" , "Another fencing sword" , "Thin fencing sword" , "Fencing sword, heavier than a foil" , "Duelling weapon" .)