Underlying principle of some food and drink (9)
I believe the answer is:
rationale
'underlying principle' is the definition.
(I've seen this before)
'some food and drink' is the wordplay.
'some food' becomes 'ration' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'and' is a charade indicator (letters next to each other).
'drink' becomes 'ale' (type of beer).
'ration'+'ale'='RATIONALE'
'of' is the link.
Can you help me to learn more?
(Other definitions for rationale that I've seen before include "Logical reasons (for action)" , "Logic, reasoning" , "Good reason" , "Basis of logic" , "Logical basis (for a course of action)" .)