Secrets penned by co-star can alarm (6)
I believe the answer is:
arcana
'secrets' is the definition.
(arcana are secrets or mysteries)
'penned by co-star can alarm' is the wordplay.
'penned by' indicates the answer is hidden within the clue.
'ARCANA' is hidden in the letters of 'co star can alarm'.
(Other definitions for arcana that I've seen before include "Secrets, mysteries" , "Tarot cards" , "Secret things" .)