Crossword Genius

Acceleration in void, once per orbit (6)

Ross

I believe the answer is:

annual

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'per orbit' is the definition.
I can't judge whether this defines the answer.

'acceleration in void' is the wordplay.
'acceleration' becomes 'a' (symbol used in Physics e.g. f=ma).
'in' means one lot of letters goes inside another.
'void' becomes 'annul' (synonyms).
'a' put within 'annul' is 'ANNUAL'.

'once' acts as a link.

Can you help me to learn more?

(Other definitions for annual that I've seen before include "Every twelve months" , "Book published yearly" , "Lasting for a year" , "(Of plant) living for a year or less" , "Twelve-monthly" .)

I've seen this clue in the Sydney Morning Herald.
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