A whole number teeing off close to marker (7)
I believe the answer is:
integer
'a whole number' is the definition.
('integer' can be a synonym of 'whole number')
'teeing off close to marker' is the wordplay.
'off' is an anagram indicator.
'close to' says to take the final letters (the close/ending of).
The final letter of 'marker' is 'r'.
'teeing' with letters rearranged gives 'intege'.
'intege'+'r'='INTEGER'
(Other definitions for integer that I've seen before include "Treeing (anag.)" , "Gin tree (anag)" , "Get Erin (anag)" , "Whole unit" , "rational number remains" .)