Do let's make changes for the firstborn (6)
I believe the answer is:
oldest
'the firstborn' is the definition.
(I've seen this in another clue)
'do let's make changes' is the wordplay.
'make changes' indicates an anagram (I've seen this in other clues).
'do'+'lets'='dolets'
'dolets' with letters rearranged gives 'OLDEST'.
'for' acts as a link.
(Other definitions for oldest that I've seen before include "Let's do! (anag.)" , "Most aged" , "Least recent" , "Eldest" , "Least young" .)