Bigger and better (7)
I believe the answer is:
greater
This clue is a double definition.
'bigger' is the first definition.
('great' can be a synonym of 'big')
'better' is the second definition.
(greater can mean better or more impressive)
'and' is the link.
(Other definitions for greater that I've seen before include "Bigger, larger" , "Larger or more outstanding" , "of more importance" , "More renowned" , "Less small" .)