small
loose
a loose flap of skin

tent
pocket
a stylish jacket with leather cuffs and pocket flaps

cat (BrE)

lift, lift back, lift up, open, pull back, push aside
He pulled back the tent flap.

close
lower
The pilot lowered the flaps as the aircraft came into land.

gently
wildly
The sails flapped wildly in the stiff breeze.

Flap is used with these nouns as the subject:
flag, sail, wing

Flap is used with these nouns as the object:
arm, wing