Intuitively, I would guess it's 「いってんさ」, but I could image 「いちてんさ」just as well.
For bonus points: is there any reference or dictionary that explains the pronunciation of numbers with counters (beyond the obvious ones taught in beginner's courses)?
I also believe it is いってんさ。
It means a one point difference (or gap)
一点、二点 etc ＝one point, two point etc
差＝difference, eg 差がある＝there is a difference or gap.
is more common than 一点差 （？）eg: