Fellow leaves donkey wilted, without energy - that's sweet! (3,4,3)

'sweet' is the definition.
I know nothing about this answer so I cannot judge whether this works.

'fellow leaves donkey wilted without energy that's' is the wordplay.
'fellow leaves donkey' becomes 'key' (I can't justify this - if you can you should give a lot more credence to this answer).
'wilted' becomes 'limp' (synonyms).
'without' indicates putting letters inside ('without' can be similar in meaning to 'outside').
'energy' becomes 'e' (physics symbol as in E=mce).
'that's' becomes 'ie'.
'limp' placed around 'e' is 'limep'.
'key'+'limep'+'ie'='KEY LIME PIE'

Can you help me to learn more?