Drink produced from honey and nuts, packed with energy (4)
I believe the answer is:
mead
'drink' is the definition.
(mead is a kind of drink)
'nuts packed with energy' is the wordplay.
'nuts' becomes 'mad' (informal term).
'packed with' is an insertion indicator.
'energy' becomes 'e' (physics symbol as in E=mc2).
'mad' placed around 'e' is 'MEAD'.
'produced from honey and' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for mead that I've seen before include "The poetic field" , "Cornish drink" , "Drink made of fermented honey and water" , "Honeyed drink for dame" , "US anthropologist, Margaret d. 1978" .)