Disciplined exercise is a boring thing (5)
I believe the answer is:
drill
This might be a double definition.
'disciplined exercise' is the first definition.
'drill' can be an answer for 'exercise' (synonyms). I am not certain of the 'disciplined' bit.
'boring thing' is the second definition.
(I've seen this in another clue)
'is a' acts as a link.
Can you help me to learn more?
(Other definitions for drill that I've seen before include "Gimlet" , "Boring machine" , "Instruct" , "primate is > bore" , "training exercise" .)