Maximum sum to invest, ultimately wasted (6)
I believe the answer is:
utmost
'maximum' is the definition.
('utmost' can be a synonym of 'maximum')
'sum to invest ultimately wasted' is the wordplay.
'ultimately' says to take the final letters.
'wasted' indicates anagramming the letters.
The final letter of 'invest' is 't'.
'sum'+'to'+'t'='sumtot'
'sumtot' anagrammed gives 'UTMOST'.
(Other definitions for utmost that I've seen before include "Farthest" , "Maximum effort" , "Extremest" , "Furthest or maximum possible" , "Furthest point or degree of effort" .)