A number turns it green (7)
I believe the answer is:
integer
'a number' is the definition.
(I know that integer is a type of real number)
'turns it green' is the wordplay.
'turns' indicates anagramming the letters.
'it'+'green'='itgreen'
'itgreen' with letters rearranged gives 'INTEGER'.
(Other definitions for integer that I've seen before include "Treeing (anag.)" , "rational number remains" , "Result of counting" , "Get Erin (anag)" , "Positive or negative whole number" .)