Someone badly missing school days (6)
I believe the answer is:
truant
'someone' is the definition.
(truant is a kind of someone)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for truant that I've seen before include "Pupil skipping school" , "Runaway" , "School leaver" , "Schoolboy dodging school" , "One is absent from school without permission" .)