Clue: Bush's grimly humorous letter from long ago (10)
Answer: BLACKTHORN
Explanation: Definition: Bush
A charade of BLACK ('grimly humorous') plus THORN ('letter from long ago'; the character Þ which results in "the" being rendered in fake antique as "ye"). The bush BLACKTHORN (which can grow into a small tree, with wood stout enough to make walking sticks and shillelagh) is also known as the sloe.