Business lease revised to include carbon in mix (8)
I believe the answer is:
coalesce
'mix' is the definition.
(synonyms)
'business lease revised to include carbon' is the wordplay.
'business' becomes 'co' (short for company).
'revised' indicates anagramming the letters.
'to include' means one lot of letters goes inside another.
'carbon' becomes 'C' (C is the chemical symbol for carbon).
'lease' with letters rearranged gives 'alese'.
'alese' enclosing 'c' is 'alesce'.
'co'+'alesce'='COALESCE'
'in' is the link.
(Other definitions for coalesce that I've seen before include "Grow together, amalgamate" , "blend" , "'Fuse, combine (8)'" , "Unite" , "Join into one mass" .)