発 is a level 9 kanji, and there are 20 vocab words on the site that use it, but none of them use the ほつ reading.
ほつ is an older reading for the kanji than はつ, so words that were imported to Japan initially came over with this reading. Later words got the はつ reading.
Since it’s older, it also appears in some Buddhist terminology I won’t bother going into here.
発作 (ほっさ) - fit, spasm, seizure
てんかん発作 (てんかんほっさ) - epileptic seizure
心臓発作 (しんぞうほっさ) - heart attack [the more medical-sounding word is 心筋梗塞 しんきんこうそく, like myocardial infarction]
発足 (ほっそく) starting, inauguration, launch, establishment
発端 (ほったん) - origin, genesis, opening, start
発起 (ほっき) - promotion, proposal
発起人 (ほっきにん) - originator, promoter
一念発起 (いちねんほっき) - being resolved to (do something); having a wholehearted intention
#1 - 空 (そら) as a prefix