Best sort of capital investment (4)
I believe the answer is:
Here is my best explanation:
'investment' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'best sort of capital' is the wordplay.
'best sort' becomes 'hal' (I am not sure about this - if you are sure you should believe this answer much more).
'capital' indicates taking the first letters.
The initial letter of 'of' is 'o'.
(Other definitions for halo that I've seen before include "Holy ring", "Aureole", "Disc of light", "Corona", "Hagiological aura featured in Christian art", "Angel's head-light", "Sign of saintliness", "Sign of holiness", "Nimbus" and "Ring of light (signifying holiness)".)