Shareholders' funds left in custody of the heads of Ernst & Young (6)
I believe the answer is:
equity
'shareholders' funds' is the definition.
(value of shares)
'left in custody of the heads of ernst young' is the wordplay.
'left' becomes 'quit' (I've seen this before).
'in custody of' means one lot of letters goes inside another (inserted letters are taken into custody by others).
'the heads of' suggests taking the first letters.
The first letters of 'ernst young' is 'ey'.
'quit' going within 'ey' is 'EQUITY'.
(Other definitions for equity that I've seen before include "Even-handedness" , "Moral justice for the actor's union" , "One trade union" , "Fair play - actors' union" , "Fairness for actors' trade union" .)