What does the cryptic crossword clue "Kind of yarn in the reply after exchange of a couple of letters (5-3)" mean?

The definition is "Kind of yarn". The wordplay uses an anagram. anagram of THE + REPLY = THREEPLY; definition: "Kind of yarn" The answer is THREEPLY.

Kind of yarn in the reply after exchange of a couple of letters (5-3)

Guardian Cryptic #28990 | 14 Across | 2023-02-10 flower