Study in school, earnestly (5)
I believe the answer is:
learn
'study' is the definition.
(I know that study is a more specific form of learn)
'in school earnestly' is the wordplay.
'in' says the answer is hidden in the clue.
'LEARN' can be found hidden inside 'school earnestly'.
(Other definitions for learn that I've seen before include "Get to know" , "Acquire knowledge or skill" , "Study or digest" , "Take in" , "Be educated" .)