First-born gets sled to play with (6)
I believe the answer is:
oldest
'first-born' is the definition.
(I've seen this in another clue)
'sled to play with' is the wordplay.
'play with' indicates an anagram (I've seen 'played with' mean this (I've seen this in other clues)).
'sled'+'to'='sledto'
'sledto' anagrammed gives 'OLDEST'.
'gets' acts as a link.
(Other definitions for oldest that I've seen before include "First in time" , "Eldest" , "Let's do! (anag.)" , "Most ancient (O, not E)" , "Most senior in age" .)