Boxer stars alongside grand old scientist (7)
I believe the answer is:
galileo
'old scientist' is the definition.
(Galileo Galilei, 1564-1642)
'boxer stars alongside grand' is the wordplay.
'boxer' becomes 'ali' (Muhammad Ali).
'stars' becomes 'Leo' (constellation).
'alongside' means one lot of letters go next to another.
'grand' becomes 'g' (G is short for grand as in a thousand).
'ali'+'leo'='alileo'
'alileo' put after 'g' is 'GALILEO'.
(Other definitions for galileo that I've seen before include "Pioneer scientist" , "He said Eppur si muove (But it does move)" , "Italian astronomer and mathematician, d. 1642" , "It. C 16 to 17 astronomer" , "Sixteenth and seventeenth century Italian astronomer tried for heresy" .)