Turnover of corporation, American, on grand scale (5)
I believe the answer is:
gamut
'scale' is the definition.
(I know that gamut is a type of scale)
'turnover of corporation american on grand' is the wordplay.
'turnover of' says the letters should be written in reverse (I've seen 'turnover' mean this (the letters turn over)).
'corporation' becomes 'tum' ('corporation' is a humorous term for stomach fat).
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'on' is a charade indicator (letters next to each other).
'grand' becomes 'g' (G is short for grand as in a thousand).
'tum'+'a'='tuma'
'tuma' reversed gives 'amut'.
'amut' put after 'g' is 'GAMUT'.
(Other definitions for gamut that I've seen before include "Entire range" , "Complete range or scale" , "Complete range (of something)" , "Whole range (from A to Z)" , "The whole range of, say emotions" .)