One may invest in more rococo houses (9)
I believe the answer is:
financier
'one may invest' is the definition.
(a financier invests in things)
'in more rococo houses' is the wordplay.
'more rococo' becomes 'fancier' ('fancy' can be similar in meaning to 'rococo').
'houses' is an insertion indicator.
'in' placed into 'fancier' is 'FINANCIER'.
(Other definitions for financier that I've seen before include "Manager of large-scale monetary resources" , "One handling money" , "I can infer (anag)" , "expert on money matters" , "City man" .)