Tiny, oddly main nucleus spinning (9)
I believe the answer is:
minuscule
'tiny' is the definition.
(I've seen this before)
'oddly main nucleus spinning' is the wordplay.
'oddly' means one should take alternating letters (letters in the odd-numbered positions are used).
'spinning' indicates an anagram.
The alternate letters of 'main' are 'mi'.
'nucleus' anagrammed gives 'nuscule'.
'mi'+'nuscule'='MINUSCULE'
(Other definitions for minuscule that I've seen before include "Not important" , "Fine" , "A lower-case letter" , "not capital" , "Extremely small, tiny" .)