A whole number one regent upset (7)
I believe the answer is:
integer
'a whole number' is the definition.
('integer' can be a synonym of 'whole number')
'one regent upset' is the wordplay.
'one' becomes 'i' (Roman numeral).
'upset' indicates anagramming the letters.
'regent' anagrammed gives 'nteger'.
'i'+'nteger'='INTEGER'
(Other definitions for integer that I've seen before include "Perhaps 60,000" , "Say one" , "Whole unit" , "One for example" , "A whole number, not a fraction" .)