How many letters does the top answer for “the offspring who came first in the order of birth” have?

The most common answer is eldest (6 letters).

Crossword clue: the offspring who came first in the order of birth

2 answers found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

eldest 6 letters wn-def

the offspring who came first in the order of birth

firstborn 9 letters wn-def

the offspring who came first in the order of birth

first in order of birth 60% similar → eldest, firstborn