Soldier, is ahead by small number (about five hundred) (8)
I believe the answer is:
commando
'soldier' is the definition.
(I've seen this before)
'ahead by small number about five hundred' is the wordplay.
'ahead by' becomes 'comma' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'small number' becomes 'no' (abbreviation for 'number').
'about' indicates putting letters inside.
'five hundred' becomes 'D' (Roman numeral for a00).
'no' enclosing 'd' is 'ndo'.
'comma'+'ndo'='COMMANDO'
'is' is the link.
Can you help me to learn more?
(Other definitions for commando that I've seen before include "shock troops" , "Member of a special service military unit" , "Soldier from elite unit" , "Soldier trained for carrying out raids" , "Royal Marine Corps soldier" .)