national
battle
burning
tattered
A tattered flag hung from the roof of the burned-out building.

chequered/checkered
Hamilton took the chequered/checkered flag to win his fourth Grand Prix of the season.

white
offside (in football/soccer, hockey, etc.)
warning (usually figurative)
The fact that it was so cheap should have been a warning flag for me.

fly, hang, hang out, hoist, raise, run up
a ship flying a Russian flag

plant
lower
wave
The crowd all waved flags as the president came past.

unfurl
carry
salute
capture
burn

hang
flap, flutter, fly, wave
a flag fluttering in the breeze

be at half mast, fly at half mast

pole (usually flagpole)

under a/the flag
a ship sailing under the Liberian flag

flag of
a flag of truce

flag of convenience
flag of surrender

Flag is used with these nouns as the subject:
energy, interest

Flag is used with these nouns as the object:
cab, taxi