Get even less to do: secret left to be cracked (6,3,6)
I believe the answer is:
settle old scores
'get even' is the definition.
(I've seen this before)
'less to do secret left to be cracked' is the wordplay.
I cannot quite see how this works, but
'do' could be 'set' (synonyms) and 'set' is found within the answer.
'left' could be 'l' (common abbreviation) and 'l' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for settle old scores that I've seen before include "Get even" , "Deal with historical complaints" .)