Secret of criminal avoiding punishment (4,4)
I believe the answer is:
scot free
'avoiding punishment' is the definition.
(I've seen this before)
'secret of criminal' is the wordplay.
'criminal' indicates an anagram.
'secret'+'of'='secretof'
'secretof' with letters rearranged gives 'SCOT-FREE'.
(Other definitions for scot free that I've seen before include "avoiding punishment" , "the reverse of what Wallace hoped to be" , "not having to pay" , "After next month a union may be so" , "It may be got off" .)