Do let's make changes for the first-born (6)
I believe the answer is:
oldest
'the first-born' is the definition.
(I've seen this in another clue)
'do let's make changes' is the wordplay.
'make changes' is an anagram indicator (I've seen this in other clues).
'do'+'lets'='dolets'
'dolets' is an anagram of 'OLDEST'.
'for' is the link.
(Other definitions for oldest that I've seen before include "First in time" , "Most aged" , "Most ancient (O, not E)" , "Least recent" , "Let's do! (anag.)" .)