Food chain: bear comes first, though not invariably top (6)
I believe the answer is:
brunch
'food' is the definition.
(brunch is a kind of food)
'chain bear comes first though not invariably top' is the wordplay.
I cannot quite see how this works, but
'chain' could be 'ch' (abbreviation for the unit of length) and 'ch' is present in the answer.
'not' could be 'un' (prefix meaning 'not') and 'un' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for brunch that I've seen before include "Combined breakfast and lunch" , "combination of meals" , "Late morning repast" , "Late-morning meal" , "Two meals in one" .)