Goal that's one in a million (3)
I believe the answer is:
aim
'goal' is the definition.
(aim is a kind of goal)
'one in a million' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' means one lot of letters goes inside another.
'million' becomes 'm' (abbreviation).
'a'+'m'='am'
'i' placed into 'am' is 'AIM'.
'that's' is the link.
(Other definitions for aim that I've seen before include "Train" , "Point (gun); intention" , "Point (a camera)" , "Steady, ... fire" , "End" .)