Endless film on the queen and sodium lake (7)

I believe the answer is:

eternal

I'm a little stuck... Click here to teach me more about this clue!

'endless' is the definition.
(I know that endless can be written as eternal)

'film on the queen and sodium lake' is the wordplay.
'film on' becomes 'et' (E.T. the Extra-Terrestrial. I am not sure about the 'on' bit.).
'the queen' becomes 'ER' (Elizabeth Regina).
'and' is a charade indicator (letters next to each other).
'sodium' becomes 'NA' (chemical symbol).
'lake' becomes 'L' (geographical abbreviation).
'et'+'er'+'na'+'l'='ETERNAL'

Can you help me to learn more?

(Other definitions for eternal that I've seen before include "carrying on endlessly" , "with lasting benefit" , "Forever or unchanging" , "'Lasting forever, unending (7)'" , "always to be remembered" .)

I've seen this clue in The Daily Express.

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