Secret of getting off without penalty (4-4)
I believe the answer is:
scot-free
'without penalty' is the definition.
(I've seen this before)
'secret of getting off' is the wordplay.
'getting off' is an anagram indicator (off can mean inaccurate or wrong).
'secret'+'of'='secretof'
'secretof' with letters rearranged gives 'SCOT-FREE'.
(Other definitions for scot-free that I've seen before include "Unharmed, not punished like kilted gent" , "Entirely without penalty or injury" , "Secret of being unpunished - like Jock?" , "'Without any punishment whatsoever (4,4)'" , "Without harm or loss" .)