First in the world! (6)
I believe the answer is:
eldest
'first' is the definition.
(similar in meaning)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for eldest that I've seen before include "Most old" , "Most advanced in years" , "Most senior of two" , "Most advanced in age, led set (anag.)" , "Most aged" .)