Exercise is a bore (5)
I believe the answer is:
drill
I believe this is a double definition.
'exercise' is the first definition.
(military exercise)
'a bore' is the second definition.
(bore can mean to drill into something)
'is' acts as a link.
(Other definitions for drill that I've seen before include "School" , "Material - exercise" , "Twill-weave cotton cloth" , "Dental assistant" , "Routine" .)