Crossword Genius

Apprentice in series, cheery at heart (7)

Ross

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" .)

I've seen this clue in The Times and The Telegraph.
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