Number one regent, surprisingly (7)
I believe the answer is:
integer
'number' is the definition.
(integer is a kind of number)
'one regent surprisingly' is the wordplay.
'one' becomes 'i' (Roman numeral).
'surprisingly' is an anagram indicator.
'regent' with letters rearranged gives 'nteger'.
'i'+'nteger'='INTEGER'
(Other definitions for integer that I've seen before include "Say one" , "rational number remains" , "Get Erin (anag)" , "one or two, perhaps" , "One for example" .)