For a number, the setter leaves four and nine out (7)

'for a' is the definition.
I know nothing about this answer so I cannot tell whether this works.

'number the setter leaves four and nine out' is the wordplay.
I cannot really see how this works, but
'number' could be 'e' (e is a kind of number) and 'e' is found within the answer.
an anagram of 'four' is 'urof' which is within the answer.
'and' could be 'n' (common abbreviation for 'and') and 'n' is located in the answer.
A single letter 'n' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).