Endlessly questioned about investment in painting (9)
I believe the answer is:
grisaille
'painting' is the definition.
(I've seen this before)
'endlessly questioned about investment' is the wordplay.
'endlessly questioned' becomes 'grille' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'about' means one lot of letters goes inside another.
'investment' becomes 'isa' (Individual Savings Account).
'grille' enclosing 'isa' is 'GRISAILLE'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for grisaille that I've seen before include "piece not in colour" , "Grey monochrome painting" .)