What does the cryptic crossword clue "What's inside is revealed by bird in nest-building (8)" mean?

The definition is "What's inside". The wordplay uses an anagram. anagram of RAIL (synonym="bird") + NEST (synonym="NEST") = ENTRAILS; definition: "What's inside" The answer is ENTRAILS.

What's inside is revealed by bird in nest-building (8)

Guardian Cryptic #26837 | 25 Across | 2016-03-21 flower

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Telegraph #28517 (2017-08-28)