Be in earnest when it comes to nasty dealings (4,8)
I believe the answer is:
mean business
'be in earnest' is the definition.
(I've seen this before)
'nasty dealings' is the wordplay.
'nasty' becomes 'mean' (similar in meaning).
'dealings' becomes 'business' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'mean'+'business'='MEAN BUSINESS'
'when it comes to' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for mean business that I've seen before include "Be deadly serious" , "usury" , "Sees man in bus (anag) - be in earnest" , "Intend to take positive action" , "underhand affair" .)