I, with knight, greet by arrangement a whole number (7)
I believe the answer is:
integer
'a whole number' is the definition.
('integer' can be a synonym of 'whole number')
'i with knight greet by arrangement' is the wordplay.
'with' indicates putting letters inside.
'knight' becomes 'n' (chess).
'by arrangement' indicates anagramming the letters.
'greet' is an anagram of 'teger'.
'n' put into 'teger' is 'nteger'.
'i'+'nteger'='INTEGER'
(Other definitions for integer that I've seen before include "Result of counting" , "rational number remains" , "One for example" , "Positive or negative whole number" , "Get Erin (anag)" .)