Apprentice in series, cheery at heart (7)
I believe the answer is:
trainee
'apprentice' is the definition.
(someone undergoing training)
'series cheery at heart' is the wordplay.
'series' becomes 'train' (train is a kind of series).
'at heart' means to look at the middle letters.
The middle of 'cheery' is 'ee'.
'train'+'ee'='TRAINEE'
'in' acts as a link.
(Other definitions for trainee that I've seen before include "A beginner in Aintree" , "encountered bad weather in driving area" , "Student" , "Apprentice" , "Job learner" .)