Small amount I invested in total for arena (7)
I believe the answer is:
stadium
'arena' is the definition.
('stadium' can be a synonym of 'arena')
'small amount i invested in total' is the wordplay.
'small amount' becomes 'tad' (I've seen this before).
'invested in' indicates putting letters inside.
'total' becomes 'sum' ('sum' can be a synonym of 'total').
'tad'+'i'='tadi'
'tadi' inserted into 'sum' is 'STADIUM'.
'for' is the link.
(Other definitions for stadium that I've seen before include "Sports ground as for athletics" , "Large structure for open-air sports" , "A major sports ground" , "eg Yankee, Wembley" , "Admit us (anag) - sports ground" .)