Happy to start just over yonder field under lindens (6)
I believe the answer is:
joyful
'happy' is the definition.
(I know that happy can be written as joyful)
'start just over yonder field under lindens' is the wordplay.
'start' suggests taking the first letters.
The initial letters of 'just over yonder field under lindens' is 'JOYFUL'.
'to' acts as a link.
(Other definitions for joyful that I've seen before include "In state of extreme gladness" , "Feeling rapture or delight" , "- - and triumphant (carol)" , "Elated" , "Happy" .)