Great soldier cross when entering a famous rowing club (9)
I believe the answer is:
alexander
'great soldier' is the definition.
(Alexander the Great)
'cross when entering a famous rowing club' is the wordplay.
'cross' becomes 'x' (X is cross-shaped).
'when entering' indicates putting letters inside.
'famous rowing club' becomes 'Leander' (Leander Club, rowing club based in Berkshire).
'a'+'leander'='aleander'
'x' inserted into 'aleander' is 'ALEXANDER'.
(Other definitions for alexander that I've seen before include "Field marshal" , "band's leader on record" , "Great king of Macedonia BC" , "A Great old Macedonian king" , "- Pope, English poet" .)