Source of easy income (5,5)
I believe the answer is:
gravy train
'source of easy income' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for gravy train that I've seen before include "Opportunity to make money" , "Situation making easy money (colloq.)" , "easy opportunity of profit" , "Scheme for easy money" , "Jobs with more perks than work" .)