Finance partners invested in needy, supporting son (7)
I believe the answer is:
sponsor
'finance' is the definition.
(I've seen this before)
'partners invested in needy supporting son' is the wordplay.
'partners' becomes 'ns' (North and South are partners in the game bridge).
'invested in' is an insertion indicator.
'needy' becomes 'poor' (similar in meaning).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'son' becomes 's' (genealogical abbreviation for son).
'ns' placed into 'poor' is 'ponsor'.
'ponsor' put after 's' is 'SPONSOR'.
(Other definitions for sponsor that I've seen before include "Event funder; godparent" , "One paying to support" , "One offering support" , "One paying for, eg, event" , "One financially supporting an activity" .)