Not worried about a heartless whistle-blower (8)
I believe the answer is:
carefree
'not worried' is the definition.
(I've seen this before)
'about a heartless whistle-blower' is the wordplay.
'about' becomes 'ca' (circa).
'a heartless' means to remove the middle letters.
'whistle blower' becomes 'referee' (I've seen this in another clue).
'referee' with its middle removed is 'refree'.
'ca'+'refree'='CAREFREE'
(Other definitions for carefree that I've seen before include "Without the burden of responsibility" , "Without a worry in the world" , "Without worry or responsibility" , "insouciant" , "Unworried, happy-go-lucky" .)