Bread and butter supplier is a steady earner (4,3)
I believe the answer is:
cash cow
'a steady earner' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'bread and butter supplier' is the wordplay.
'bread' becomes 'cash' (I've seen this before).
'and' says to put letters next to each other.
'butter supplier' becomes 'cow' (I can't explain this - if you can you should believe this answer much more).
'cash'+'cow'='CASH COW'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for cash cow that I've seen before include "Product or business making easy money" , "E.g. business milked for money" , "Money-spinning business" , "Saw choc (anag.); steady investment" , "troublefree business?" .)