Number turns it green (7)
I believe the answer is:
integer
'number' is the definition.
(I know that integer is a type of number)
'turns it green' is the wordplay.
'turns' indicates an anagram.
'it'+'green'='itgreen'
'itgreen' with letters rearranged gives 'INTEGER'.
(Other definitions for integer that I've seen before include "Say one" , "A whole number, not a fraction" , "One for example" , "Gin tree (anag)" , "Get Erin (anag)" .)