The first clue describes a word, which, when F is added as a prefix changes to a word meaning the second clue. It is like: first clue + F (as a prefix) = second clue.
-1.A unit of measurement of length with F becomes a small singing bird.
-2.To be windy with F becomes a legendary being, loved by small children.
-3.F with something not wrong becomes extreme fear.
-4.F with a pointed weapon, often used with a bow becomes a litter of pigs.
-5.To write on glass with F becomes a verb meaning 'to bring'.
-6.A night bird with F becomes chicken reared for meat.
-7.To seal with F becomes a group of animals.
-8.Eternity with F becomes a common ailment.
9.To employ with F becomes a verb meaning ' to mix'.
-10.F with something unbendable becomes extremely cold.