Cold criminal has neat source of steady income (4,3)
I believe the answer is:
cash cow
'source of steady income' is the definition.
'cash cow' can be an answer for 'source' (I have seen 'source of profit' mean 'cash cow' so perhaps 'source' could also mean 'cash cow'). I'm not sure about the 'of steady income' bit.
'cold criminal has neat' is the wordplay.
'cold' becomes 'c' (eg on taps).
'criminal' indicates an anagram.
'neat' becomes 'cow' (I can't justify this - if you can you should give a lot more credence to this answer).
'has' is an anagram of 'ash'.
'c'+'ash'+'cow'='CASH COW'
Can you help me to learn more?
(Other definitions for cash cow that I've seen before include "regular provider of income" , "troublefree business?" , "source of profit" , "Saw choc (anag.); steady investment" , "Business making large profits" .)