Clue: Liz eats chocolate biscuit; it's 'to be found within' (7)
Answer: BETWIXT
Explanation: Definition: within
An envelope ('eats') of TWIX (trade name, 'chocolate biscuit' – a chocolate covered biscuit, more or less) in BET ('Liz' – both abbreviated versions of Elizabeth). The rest of the clue seems superfluous.