Best sort of capital investment (4)
I believe the answer is:
halo
'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'.
'hal'+'o'='HALO'
Can you help me to learn more?
(Other definitions for halo that I've seen before include "Disc of light" , "Saint's overhead ring of light" , "Hagiological aura featured in Christian art" , "Aura round admired person" , "Sign of sainthood" .)