Profit: billion, and rising as well (4)
I believe the answer is:
boot
'profit' is the definition.
(thesaurus)
'billion and rising as well' is the wordplay.
'billion' becomes 'b'.
'and' means one lot of letters go next to another.
'rising' says the letters should be written in reverse (going up in a down clue).
'as well' becomes 'too' ('too' can be a synonym of 'as well').
'too' back-to-front is 'oot'.
'b'+'oot'='BOOT'
(Other definitions for boot that I've seen before include "Profit" , "Clothing; kick" , "Sturdy shoe; start (a computer)" , "Item of footwear" , "Car luggage space; kick" .)