Speaking about planetary motion no little amount (4)
I believe the answer is:
oral
'speaking' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'about planetary motion no little amount' is the wordplay.
I cannot quite understand how this works, but
'about' could be 'a' (abbreviation in Chambers) and 'a' is found in the answer.
'no' could be 'o' (resembles 0 - 'no' can mean 'zero') and 'o' is found within the answer.
'little' could be 'l' (abbreviation) and 'l' is present in the answer.
'amount' could be 'r' (R is an example) and 'r' is found within the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for oral that I've seen before include "'Spoken, not written (4)'" , "Exam using speech" , "Of the voice" , "By word of mouth, say exam" , "Ant; tourist (dial.)" .)