One, ten and a hundred divide precisely into three or so (9)
I believe the answer is:
inexactly
'or so' is the definition.
'one ten and a hundred divide precisely into three' is the wordplay.
I cannot really see how this works, but
'one' could be 'i' (Roman numeral) and 'i' is present in the answer.
'ten' could be 'x' ('X' can be a synonym of 'ten') and 'x' is found within the answer.
'and' could be 'n' (common abbreviation for 'and') and 'n' is located in the answer.
'a' is present in the answer.
'hundred' could be 'c' (Roman numeral) and 'c' is located in the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for inexactly that I've seen before include "Imprecisely" , "Not quite accurately" , "not true to form" , "Without precision" , "Tax nicely (anag.)" .)