One good friend entered and left finally happy and high-spiritedly (10)

'left finally happy and high-spiritedly' is the definition.
I can't tell whether this defines the answer.

'one good friend entered' is the wordplay.
'one' becomes 'an' (an apple is one apple).
'good' becomes 'idly' (I can't explain this - if you can you should give a lot more credence to this answer).
'friend' becomes 'mate' (mate is a kind of friend).
'entered' indicates putting letters inside.
'idly' enclosing 'mate' is 'imatedly'.
'an'+'imatedly'='ANIMATEDLY'

'and' acts as a link.

Can you help me to learn more?