Essential dietary element forming the nucleus of lion feed (6)
I believe the answer is:
iodine
'essential dietary element' is the definition.
'iodine' can be an answer for 'element' (iodine is a kind of element). I am not certain of the 'essential dietary' bit.
'the nucleus of lion feed' is the wordplay.
'the nucleus of lion' becomes 'io' (I can't justify this - if you can you should give a lot more credence to this answer).
'feed' becomes 'dine' (I've seen this before).
'io'+'dine'='IODINE'
'forming' acts as a link.
Can you help me to learn more?
(Other definitions for iodine that I've seen before include "Mother-of-pearl" , "Disinfectant made from seaweed" , "Solution of a halogen in ethyl alcohol used on wounds as an antiseptic" , "Element existing in vapour and crystalline form" , "Black/violet element" .)