乘隙而入 is an idiom

乘隙而入chéng xì ér rù

趁着虚弱疏漏的地方进入。

Origin 《去笈七签》卷一二０：“将至所居，自后垣乘虚而入，径及庭中。”

Word breakdown for “乘隙而入”: Open in Reader →

1.to seize an opportunity2.to exploit (a loophole)

而HSK 4

1.and2.as well as3.and so4.but (not)+4 more +5 more

入HSK 6

1.to enter; to go into2.to join; to become a member of3.(bound form) to conform to (as in 入时[rù shí])4.abbr. for 入声[rù shēng]+1 more +2 more