Ill-gotten gains added to the stock (5)
I believe the answer is:
graft
'stock' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'ill-gotten gains added to the' is the wordplay.
'ill-gotten gains' becomes 'graf' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'added to' is a charade indicator (letters next to each other).
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'graf'+'t'='GRAFT'
Can you help me to learn more?
(Other definitions for graft that I've seen before include "Hard work; bribery" , "Make money from corruption" , "Hard work - illicit gain" , "Surgical transplant" , "Hard work; corruption" .)