Acceleration in void, once per orbit (6)
I believe the answer is:
annual
'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" .)