Stargazer initially gazing at 51 stars (7)
I believe the answer is:
galileo
'stargazer' is the definition.
(Galileo Galilei)
'initially gazing at 51 stars' is the wordplay.
'initially' indicates taking the first letters.
'51' becomes 'LI' (Roman numerals).
'stars' becomes 'leo' (constellation).
The initial letters of 'gazing at' is 'ga'.
'ga'+'li'+'leo'='GALILEO'
(Other definitions for galileo that I've seen before include "Famous Italian" , "Sixteenth and seventeenth century Italian astronomer tried for heresy" , "Refracting telescope inventor" , "Great Italian astronomer and physicist of 17th century, suppressed by Church" , "Italian scientist and astronomer" .)