Nobleman in Paris that finally invests in planet (8)
I believe the answer is:
marquess
'nobleman' is the definition.
(I know that marquess is a type of nobleman)
'in paris that finally invests in planet' is the wordplay.
'in paris that' becomes 'que' (French for 'that').
'finally' suggests the final letters.
'in' is an insertion indicator.
'planet' becomes 'mars' (Mars is a planet).
The last letter of 'invests' is 's'.
'que'+'s'='ques'
'ques' put within 'mars' is 'MARQUESS'.
(Other definitions for marquess that I've seen before include "He's entitled" , "Lady of nobility" , "noble" , "Nobleman" .)