One side of your collar is tucked into your sweater
One side of your collar is tucked under the neckband of your sweater
「襟が(片方だけ)入っちゃってる」= One side of your collar is tucked into your sweater / One side of your collar is tucked under the neckband of your sweater
ボキャブラリー
one side = 片方
collar = 襟
tucked into = 入っちゃっている
sweater = セーター
neckband = セーターやTシャツなどの首のところ