Twist cap in alarm (5)
I believe the answer is:
panic
'alarm' is the definition.
(alarm can mean panic or worry)
'twist cap in' is the wordplay.
'twist' indicates an anagram (a twisted or distorted version of the letters).
'cap'+'in'='capin'
'capin' anagrammed gives 'PANIC'.
(Other definitions for panic that I've seen before include "much unease" , "Horror" , "Uncontrollable terror, infectious fear" , "Lose one's cool" , "Sudden uncontrollable fear" .)