Old scientist gives girl one sign (7)
I believe the answer is:
galileo
'old scientist' is the definition.
(Galileo Galilei, 1564-1642)
'girl one sign' is the wordplay.
'girl' becomes 'gal' ().
'one' becomes 'i' (Roman numeral).
'sign' becomes 'leo' (star sign).
'gal'+'i'+'leo'='GALILEO'
'gives' acts as a link.
(Other definitions for galileo that I've seen before include "Astronomer - ogle Ali (anag)" , "Famous scientist" , "Italian scientist and astronomer" , "Italian astronomer who had trouble with church" , "Famous Italian" .)