What does the cryptic crossword clue "Young creature to plead insanity (7)" mean?

The definition is "Young". The wordplay uses an anagram. anagram of TO + PLEAD = TADPOLE; definition: "Young creature" The answer is TADPOLE.

Young creature to plead insanity (7)

Guardian Cryptic #27371 | 28 Across | 2017-12-04 flower