Prince happiest flying in training period (14)
I believe the answer is:
apprenticeship
'training period' is the definition.
'prince happiest flying' is the wordplay.
'flying' indicates an anagram.
'prince'+'happiest'='princehappiest'
'princehappiest' with letters rearranged gives 'APPRENTICESHIP'.
'in' is the link.
(Other definitions for apprenticeship that I've seen before include "Period for learning a skilled trade" , "articles" , "Training" .)