請問，They want to find a better place to hold a party in. 這句若把後面的in拿掉語意還能通順嗎？

in 的需要,是因為a better place,而不是hold的關係.

這句話 是從"hold a party in a better place"變化而來的.

所以,in這個介係詞 是不可省略的.但是在口頭對話中,卻很多的人不說in.還是可以被聊解的!

According to the logic of meaning, the sentence should be rewritten as:
They want to find a better place to hold 'the" party in.

"to hold the party in" 這不定式短語 (infinitive phrase) functions as adjective to modify the noun phrase "a better place".
It is equivalent to:
- a better place that we should/could hold the party in
- a better place in which we should/could hold the party
- a better place where we should/could hold the party

Therefore, the preposition "in" cannot be omitted in the above sentence.
In spoken English, people often omit the preposition, such as:
- a better place to live; a better place to travel, etc.
We could also use prepositional phrase to modify the noun in this case:
- They want to find a better place for (holding) the party.

題外話:
They want to find a place to hold a party. (without preposition "in")
"to hold a party" can be interpreted to function as adverb to modify "find" as the purpose of finding a place..

Hold in 掌控，是片語動詞當及物使用，而介詞可置前或後，所以寫成hold in a party 亦可，若將in去掉，是舉刅的意思，視你要表達何意決定

hold a party in=hold something in
=check; restrain; hold in your temper; hold yourself in, control your feelings at a false accusation in a party with cruel members because of underserved blame to you !!!???
hold a party =To plan and have a party Throw a party.