Give firm assurance, investing millions (10)
I believe the answer is:
compromise
'give' is the definition.
(give can mean to compromise in an argument)
'firm assurance investing millions' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'assurance' becomes 'promise' (I've seen this before).
'investing' means one lot of letters goes inside another.
'millions' becomes 'm' (abbreviation).
'co'+'promise'='copromise'
'copromise' enclosing 'm' is 'COMPROMISE'.
(Other definitions for compromise that I've seen before include "Give-and-take" , "Trade-off" , "Settlement" , "Agree a middle way" , "accommodation" .)