What does the cryptic crossword clue "Trouble in Reading is entrenched (9)" mean?

The definition is "entrenched". The wordplay uses an anagram. anagram of IN + READING = INGRAINED; definition: "entrenched" The answer is INGRAINED.

Trouble in Reading is entrenched (9)

Guardian Cryptic #23996 | 6 Down | 2007-02-09 flower