Isn't worried by insect's heartbeat (7)
I believe the answer is:
instant
'heartbeat' is the definition.
'isn't worried by insect's' is the wordplay.
'worried' is an anagram indicator.
'by' is a charade indicator (letters next to each other).
'insect' becomes 'ant' (ant is a kind of insect).
'isnt' with letters rearranged gives 'inst'.
'inst'+'ant'='INSTANT'
(Other definitions for instant that I've seen before include "This coffee is made from powder" , "A second - coffee, maybe" , "type of coffee" , "Immediate; flash" , "Short moment" .)