Use up our large investment on fabulous things (9)
I believe the answer is:
splendour
'investment on fabulous things' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'use up our large' is the wordplay.
I cannot quite understand how this works, but
'large' could be 'l' (eg in clothes sizes) and 'l' is found within the answer.
'use' could be 'spend' (spending is a kind of using) and 'spend' is present in the remaining letters.
The remaining letters 'our' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for splendour that I've seen before include "Grandeur" , "Spectacle" , "Sumptuous display - Len's proud (anag)" , "Magnificent appearance" , "Brilliance or pomp" .)