Learner, more or less idlest, periodically (5)
I believe the answer is:
cadet
'learner' is the definition.
(cadet can mean a trainee)
'more or less idlest periodically' is the wordplay.
'more or less' becomes 'ca' (I can't explain this - if you can you should believe this answer much more).
'periodically' means one should take alternating letters (letters must be selected 'every so often').
The alternate letters of 'idlest' are 'det'.
'ca'+'det'='CADET'
Can you help me to learn more?
(Other definitions for cadet that I've seen before include "Apprentice officer" , "not the heir" , "Officer in training" , "A trainee army officer" , "Younger son" .)