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)?
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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.
てんさ／点差 | the difference in points
is more common than 一点差 （？）eg:
2点の点差で負けた｜We lost the game by two points.
ライオンズは点差を広げた｜The Lions widened their lead.
I believe it's
いってんさ as you said.