Is back for one last bit of private investment (5)
I believe the answer is:
siege
'investment' is the definition.
(investment can mean a siege)
'is back for one last bit of private' is the wordplay.
'back' says the letters should be written in reverse.
'for one' becomes 'eg' (both can mean 'for example').
'last bit of' indicates one should take the final letters.
The last letter of 'private' is 'e'.
'is' written backwards gives 'si'.
'si'+'eg'+'e'='SIEGE'
(Other definitions for siege that I've seen before include "Act of surrounding a town or fortress" , "Action that isolates and attacks" , "Surrounding and blockading of town by military" , "Operation to capture a fort, for example" , "Military operation cutting off access" .)