Training part that's boring (5,3)
I believe the answer is:
drill bit
'that's boring' is the definition.
(a drill bores into things)
'training part' is the wordplay.
'training' becomes 'drill' (drill is a kind of training).
'part' becomes 'bit' (both mean a piece of something).
'drill'+'bit'='DRILL BIT'
(Other definitions for drill bit that I've seen before include "Borer" , "that bores" , "it can be boring" , "tool" .)