、、。
Ahahaha, you really are bad at keeping secrets, aren't you.
Literal
Ahahaha, reeeally, secret do [nominalizer-の] [subject-が] bad-at [explanatory-なん] is [confirmatory-ね].
Very casual dialogue. ほんっと is a spoken intensive of ほんと/本当 with a small っ inserted in the middle for emphasis — 'reeeeally' in English equivalent. This 'midword gemination' (inserting a small っ for emphasis) is a common spoken feature: すっごい, めっちゃ, ちょっ. 隠し事 ('secret,' literally 'hidden thing/matter') is the standard word for something one is hiding. 苦手 is a な-adjective meaning 'bad at,' 'not good with' — takes が for its object-like argument, same as 好き or 上手. なんだね combines the explanatory なんだ with the confirmatory ね, ending on the drawn-out ~ to signal easygoing, teasing tone.