Clue: Gain nothing in this quarter? Be selective (6)
Answer: WINNOW
Explanation: Definition: Be selective
 WIN(to gain/to acquire as a result of a contest) + [ O(letter representing 0/nothing) **contained in**(in) NW(abbrev. for "northwest", the top left quarter of the grid in which this answer is) ].
Defn: …/pick out from a group of things, those you desire and rejecting the rest.