Clue: Two elements essential to our existence (and to 16 across?) (4)
Answer: LIFE
Explanation: Definition: and to 16 across?
LI + FE – Lithium and Iron: is Lithium "essential"? Wikipedia says "Trace amounts of lithium are present in all organisms. The element serves no apparent vital biological function, since animals and plants survive in good health without it". The definition is a reference to the title of the thematic film