Possibly 3 million neural folds (7)
I believe the answer is:
numeral
'possibly 3' is the definition.
(3 is an example)
'million neural folds' is the wordplay.
'million' becomes 'm' (abbreviation).
'folds' is an anagram indicator (I've seen 'folded' mean this).
'm'+'neural'='mneural'
'mneural' with letters rearranged gives 'NUMERAL'.
(Other definitions for numeral that I've seen before include "Symbol representing one or two, perhaps?" , "Such as 5, 9, iii, etc" , "one involved in maths?" , "Eg, 5, nine or 6301" , "Counting symbol" .)