Each clue answer can be split into two parts, one consisting of letters in the first half of the alphabet (sooner), the other of letters in the second half (later). The coloring for each answer entry reflects which half is which. The letters immediately before the split on each answer spell UNFLEDGED NESTLING PIGEON, which is the clue for the final blank. This answer is SQUAB.

Mountain pass on Interstate 90 (10)	SNOQUALMIE
City on the Nidelva (9)	TRONDHEIM
Winter visitor (4 5)	JACK FROST
1996 Natalie Merchant hit (8)	JEALOUSY
Disease usually contracted by children (7 3)	CHICKEN POX
Not coniferous (9)	DECIDUOUS
Louis Gossett Jr. boxing flop (9)	DIGGSTOWN
With “modern,” a design movement (3-7)	MID-CENTURY
American Chinese pre-meal selection (3 4 4)	EGG DROP SOUP
Bernd Brunnhofer game (5 3)	STONE AGE
Tom who played Loki (10)	HIDDLESTON
Hydrogen cyanide (7 4)	PRUSSIC ACID
Catch-y team-building activity (5 4)	TRUST FALL
Pokémon #069 (10)	BELLSPROUT
February subject of study (5 7)	BLACK HISTORY
Cavalier’s opponent (9)	ROUNDHEAD
Open to interpretation (9)	AMBIGUOUS
2010 Washington/Pine thriller (11)	UNSTOPPABLE
Olympic racquet sport (9)	BADMINTON
Family Ties actor (7 5)	MICHAEL GROSS
Old receptacle for waste (7 3)	CHAMBER POT
Cruise ship stop (4 2 4)	PORT OF CALL
Four-ingredient dessert (traditionally) (5 4)	POUND CAKE
(5)	SQUAB